Hi,
Continuing with the list for the applications compatible with your Iphone. By which you can use voip in your Iphone and make internet based voip calls, cut down your phone bills.
Truphone : It is been around since earliest days of mobile VoIP. The Truphone iPhone application enables you to make calls on your iPhone over the internet at very low rates. It’s brilliant for making international calls from your home country and for making calls when you’re travelling in another country. Truphone lets you see (and search) contacts stored in the iPhone Address Book, you can easily see your Truphone account balance as well as view recent calls and favourite contacts.
They even give you £1$1 FREE CREDIT to get you started. All the installation guide, with videos can be found here.
Phonegnome : PhoneGnome pays for calls between PhoneGnome members so there is no cost to you. iPhoneGnome is a web app for the iPhone. It lets you call any of your PhoneGnome contacts, anyone using a SIP-based service or on FWD, MSN, Yahoo! and GTalk.
Jajah : It is one of the oldest voip service. It is also a web based calling service. The JAJAH web app looks just like your standard iPhone dialing pad. Upon visiting the JAJAH web app in iPhone’s Safari browser, you enter your friend’s number and a local telephone number you have access to, then JAJAH will call you and your friend and connect you.