UPDATED 28 July 2008 after prodding by the first comment.
ONE OF THE BEST iPhone applications available for immediate download comes from Truphone. This is the first low-cost VoIP calling service for the iPhone. I have used it with Nokia mobile phones for nearly four months and the service has saved me at least €100 a month because when I'm roaming, I use Truphone instead of roaming voice services. And when I'm in a wifi hotspot, I toggle Truphone "on" so I can make free phone calls and send free text messages. Call quality is superb and is normally better than Skype. The only issue I have with the service is that I have been dumped off Truphone after 10-15 minutes several times, both while using it via 3G and wifi points. However, since my average call lasts no longer than five minutes, I don't encounter the problem frequently.
Get Free Calls When VoIP End-to-End. The only way Truphone is actually free is when both the caller and the called are using internet phone capability. For example, I can call another person who is running Truphone on their mobile phone in another country and my call will ring from my mobile handset to my wifi router, cross the internet to another wifi router where the other end of the call rings on a Truphone mobile phone.
Get Cheap Calls When Calling Skype-In. I have called people for free by using my mobile phone and Truphone over O2 broadband to ring them over Skype. I have programmed their Skype-In numbers on my Truphone handset (a Nokia E90). When I ring them via Skype-In, their Skype phone rings. They have to have set up their Skype account to ensure they get calls directed to their Skype software when someone calls their Skype-In number. This has also worked for people who divert their Skype-In calls to their DECT cordless Skype phones in their homes.
Call SIP Numbers for Free. I have called several people for no cost by dialing their internet phone numbers. Both people had Blueface accounts and I simply asked my Nokia E90 to ring them with an internet phone call. Truphone voice quality is exceptionally good--better than GSM to SIP--when I use the "internet phone number" on my Nokia E90 contact details to ring around.
I intend to report back on Truphone's effectiveness while roaming around Ireland. At the moment, this VoIP capability has transformed my use of the Nokia E90, making it significantly less expensive to use during the business day and saving me the cost of overnight accommodation while traveling in other cities around Europe.
Conor O'Neill gives it 4 of 5 stars.
Walt Mossberg -- "Ten iPhone Programs to Check Out"
Mike Arrington -- VoIP in my iPhone"
Hanna Sistek on CNET -- "Truphone's VoIP app dials up iPhone"