RingFree helps you connect your existing phone services to your iPhone. With RingFree you can use the SIP, Voice over IP, PBX and/or IP-based phone systems you already have from your iPhone. Also make international calls on your iPhone for the same great rate rates you get from your home phone. And make free IP-based calls (GoogleTalk, Yahoo!, MSN) from your iPhone.

Connect calls using your office’s PBX system even when you’re out and about. Your calls will look like they come from your office on the receiver’s caller ID. Plus, the calls will get billed to your business.

The first month is free and the regular rate is $30/year. Try with TestiPhone