Business Week today has a good summary of various mobile VoIP services that allow you to make phone calls from your cellphone over the Wi-Fi network. These apps are incredibly popular amongst people who use iPhones, thanks to services such as Truphone and Gorilla Mobile, while others like iCall will soon join the party. And that is just the beginning.

By 2011 the number of mobile VoIP users around the world may rise to 100 million from 7 million in 2007, according to ON World, a consulting firm based in San Diego. ON World estimates that in 2011, mobile VoIP voice services may generate $33.7 billion, up from $516 million in 2006, the most recent year for which the figure is available.

WiFi-based voice calls may not appear to make much sense in this era of $99, flat-rate unlimited plans, but when you have to make a lot of international long distance calls, the cost saving are humongous. I use Truphone on my iPhone and/or my Nokia e71 to make calls to my family and friends in India as well as to my sources, which are spread across the world.

There are times when I have used Skype (via iSkoot) on my mobiles to make calls, but Truphone is my service of choice. I save a lot of money when compared to what calls cost on the AT&T network.

No wonder phone companies don’t have much use for Wi-Fi unless they can use it off-load calls from the precious wireless spectrum to the Wi-Fi network. Others, like T-Mobile USA, have come up with a way to measure calls made via Wi-Fi hotspots and count them against wireless subscriber minutes, unless you sign up for an additional $10-a-month plan, Business Week notes.

So obviously they’ve gone out of their way to neuter VoIP services like Truphone. More recently, Nokia decided to take out VoIP functionality from some of its N-Series devices, like the N78 and the forthcoming N96, in what could very well be called carrier arm-twisting.

I think that if carriers want to compete with mobile VoIP they need to lower their long-distance prices to that of VoIP services. By using their network backbones they can offer convenience and quality to trounce the upstarts. The problem is that wireless companies will not make this logical move — until it’s too late.

Related Stories:

* 7 Ways to VoIP From Your Mobile Phone.
* iSkoot, Not Skype, Launched on Symbian.

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Nokia, the leading handset maker, has been a favorite of ours for two reasons –- it ruthlessly promoted and added Wi-Fi connectivity to its handsets and added VoIP functionality to its devices.

Carriers did not view these technology developments too kindly since it prevented them from extorting exorbitant amounts of cash for costly long distance connections, leading to the rise of mobile-VoIP players such as Truphone, Fring and Gizmo Project.

But now the Finnish giant seems to have developed cold feet, and some of its new handsets, such as the new N78, are not VoIP compatible anymore. Many of these phones are not on the list of Nokia’s VoIP compatible handsets. A reader tipped us off about this apparent change in the latest N-series phones.

…the N78 (and also to affect the forthcoming N96) which is that Nokia has very quietly and seemingly sneakily redacted their built-in VoIP / SIP implementation in all phones that come with Symbian Series 60 3rd generation Feature Pack 2 (otherwise abbreviated as S60 3.2).

I have a N78 lying around so I decided to test it myself. And lo-and-behold none of the VoIP services I am accustomed to using worked. Truphone and Gizmo are two services I typically use and neither of them work.

Ditto for Fring, a VoIP-IM service as well. However, all three worked on the Nokia E71 smart phone. When I asked Nokia if this was true, the company sent me this response, which pretty much admits that is the case, though it didn’t say why.

Nokia Nseries is committed VoIP services as part of its offering. That is why we have included SIP stack and improved the developer VoIP offering in S60 3rd Edition Feature Pack 2 by enhancing the VoIP APIs to improve the call quality, as an example.

A Nokia VoIP client is not included with the Nokia N78 and the Nokia N96 and VoIP solutions based on this particular client such as Gizmo will not work. However, Forum Nokia will cooperate with third-party developers to support them in porting their applications from S60 3.0/3.1 releases to S60 3.2. One example is Fring, whose popular application will be offered via Nokia’s Download! service for the Nokia N96.

Truphone isn’t waiting around for Nokia to do something. A company spokesman told us: “From Truphone’s perspective Nokia has removed the VoIP client from all the N-Series phones for the planned future. We are putting in a replacement client functionality so that existing customers are not orphaned.”

The theory is that Nokia isn’t mucking with E-series devices because they are more enterprise focused. Since VoWLAN is more popular with the corporations, Nokia can’t afford to remove the VoIP functionality. It is one feature that makes the phones more competitive with say Blackberry.

On the consumer front, however, voice-over-WiFi has become a thorn in the side of carriers, as exemplified by actions of carriers such as T-Mobile against VoWiFi-startups such as Truphone. Furthermore, the emergence of 3G has made it easier to route calls over the 3G network.

Funnily enough, the decision to back away from built-in VoIP comes at a time when fixed mobile convergence is finally beginning to gain traction, especially in Europe and Asia. In the U.S., Nokia launched a handset that works with T-Mobile’s Hotspot@Home service.

Nokia has to be taking a lot of heat from carriers over making VoIP easy on its devices. Whichever way you look at it, I think it is a bone-headed move by the company, which should be trying to out-innovate its competitors and be more open in terms of its features.

The decision also brings into question company’s new mantra of being open and open-source friendly. Being open isn’t about releasing some software in open source, but it is about having an open mind. Shutting down a much loved VoIP feature isn’t exactly the right move.

As our reader very aptly wrote:

….does this move by Nokia really appear to be the type of move that is indicative of a culture shift towards open source per the Symbian Foundation? Google is already culturally rooted in open source (its entire infrastructure runs on Linux clusters). I am not so confident about Nokia’s ability to shift to open source…

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While the emergence of VoIP, or voice-over-Internet protocol, technology has already helped push down the cost of making a phone call, now it’s starting to have a deflationary impact on the world of mobile, where call charges remain stubbornly high.

In the meantime, the ongoing adoption of 3G broadband and the inclusion of Wi-Fi in many high-end phones is drawing a growing amount of attention to mobile VoIP services. Indeed, research firm Disruptive Analysis predicts that the number of VoIP-over-3G users will top 250 million by the end of 2012 — from virtually zero in 2007.

We at GigaOM are constantly tinkering with these mobile services, so we’ve put together a list of seven mobile VoIP apps that we think you’ll find handy.

Skype Options

Service: Skype Mobile
Platform/Network: Java-based application that works on 50 popular phones from Nokia, Sony Ericsson, Motorola and Samsung. Can be used on numerous cellular data networks.
Cost: Free
Features: Chatting (including with a group), presence settings (offline, online, do not disturb), and Skype-to-Skype calls (including SkypeIn).
Our Opinion: If your phone is supported, Skype Mobile is a great way to add Skype chatting and calls to it. Though it would be nice if the Skype application weren’t written in Java, as these applications are often sluggish and unstable.

Service: 3 Skypephone
Platform/Network: Uses a specialized handset. Currently available in the UK, Italy, Austria, Hong Kong, Australia, Ireland, Denmark and Sweden.
Cost: The phone costs £49.99 (about $98) and can be used on a pre-paid basis. Calls cost nothing if they’re made from Skype.
Features: Free Skype-to-Skype mobile calls and the ability to conduct Skype IM conversations, all without touching your computer.
Our Opinion: The 3 Skypephone is best suited for those who need a few monthly mobile minutes and wish to talk/IM to their Skype friends along the way. The pre-paid feature makes it an attractive and inexpensive option for Skype chatters in the geographies it serves.

Service: iSkoot
Platform/Network: Mobile handsets such as BlackBerry, Nokia, Windows Mobile and Palm OS models. Also works on GSM networks.
Cost: Modest to expensive, based on usage. Because iSkoot is a hybrid VoIP/GSM service, it uses SMS and mobile minutes when making and receiving calls or Skype IM messages.
Features: SkypeIn and Skype-to-Skype calls, Skype IM messages.
Our Opinion: iSkoot is a good option for keeping in touch with your Skype contacts. However, I would look at other software applications that just use data to send Skype SMS and Skype voice traffic.

Mobile VoIP Players

Service: Truphone
Platform/Network: Nokia handsets
Cost: Incoming free calls while on the Truphone network, low per-minute rates while on a GSM network. Outgoing calls are billed at very low per minute rates.
Features: Truphone offers free calls, SMS and voice mail while logged into the Truphone network via Wi-Fi. Otherwise, Truphone forwards calls to your mobile handset and you pay a low per-minute charge.
Our Opinion: With its smart forwarding options, Truphone is particularly useful for international travelers. Whether you’re on Wi-Fi or just your normal GSM network, you can be reached via your Truphone number no matter where you are (charges apply in certain cases, see their site for details). For times when Wi-Fi is not available, Truphone just released Truphone Anywhere, which utilizes local gateways for outgoing calls at low per-minute charges. I have trialed the service by forwarding calls from my Truphone number to my cell phone and the call quality was fantastic; voices were indistinguishable from any other cell phone call. A Truphone-to-Truphone VoIP call yielded even higher voice quality.

Service: Fring
Platform/Network: Nokia/Symbian handsets, Widows Mobile, iPhone (pre-release beta)
Cost: Free
Features: Allows you to make VoIP calls on any SIP network, Skype or to other Fring users. Additionally, Fring is a multi-protocol IM client that will allow you to chat with your buddies on Skype, MSN, ICQ, Google Talk, Twitter, AIM and Yahoo.
Our Opinion: I have used Fring to make SIP and Skype voice calls, and over EDGE the call is choppy and hard to understand. However, Wi-Fi provides enough bandwidth to make Fring calls clear and understandable. I wouldn’t say the quality is fantastic, but it is very comparable to a normal cell phone call.

Service: Talkonaut
Platform/Network: Available for Java-based phones, Symbian and Windows Mobile
Cost: Free
Features: Talkonaut offers free VoIP and IM chatting. For instant messaging, the application supports Google Talk, ICQ, AIM, and Yahoo. Talkonaut can also use SIP for VoIP calling.
Our Opinion: Talkonaut is a Russian offering and is still very new. The application was quite unstable on my Nokia N82 handset. With iffy performance and a very rusty user interface, I would recommend looking at other applications for this functionality, namely Fring.

Service: Nimbuzz
Platform/Network: Java program, Symbian; an iPhone version is coming soon.
Cost: Free
Features: Allows you to engage in IM conversations and conduct VoIP calls, as well as to share media such as photos and video. Nimbuzz also allows for client-to-client calls and has widgets enabling calls to originate from Facebok and MySpace. Compatible with Skype, Google Talk, AOL Instant Messenger, Windows Live Messenger, Yahoo, MySpace, Facebook and Jabber IM networks.
Our Opinion: The Symbian application is very responsive and easy to use. A VoIP call originating from my Facebook page to Nimbuzz over a Wi-Fi connection sounded good — just as good as any call over a GSM network.

Service: Gizmo5 (formerly Gizmo Project)
Platform/Network: Nokia Symbian handsets, BlackBerry, Windows Mobile, Nokia Tablets
Cost: Free for VoIP calls, low per-minute charge to call landlines and to SMS to mobiles.
Features: Gizmo5 allows you to conduct voice calls to other Gizmo5 users and grants IM conversations with friends on Gizmo5, MSN, AIM, Yahoo and Jabber.
Our Opinion: Gizmo5 is a great competitor to Skype, just not as established or well-known. They have wisely developed their client software for many platforms including numerous mobile phones, and on both Mac and PC. VoIP call quality is stellar when calling between clients, as well as to landline/mobile phones.