Back in October at the CTIA conference, AT&T announced that it would open up its 3G network to VoIP applications on the iPhone. But it was still up to Apple let to these apps into the App Store. Well, several months have gone by, and it appears that the company is finally taking action with an SDK update that will allow developers to take advantage of this function.

This is a major change from Apple's previous agreement with AT&T which only allowed VoIP communications over a Wi-Fi connection. In a press release issued late yesterday, iCall broke the news and claimed to be first and only VoIP application available for the iPhone platform that allows use over 3G networks – at least for now. With the restriction lifted, we expect Skype, Truphone and other similar calling apps to get on board soon.

Currently, iPhone voice plans from AT&T start at $39.99 and go up to $199, while unlimited data will set you back an additional $30. In theory, this means you could cut your bill to around $80 a month by sticking to the cheapest voice plan and using iCall's $9.95 Total Access plan for unlimited local calls and cheaper long distance. The possibility of free Skype-to-Skype calling over 3G sounds really tempting as well.