Once you jailbreak your phone you can install the free sip voip softphone
Here is the 5-minute video tutorial. We hope this will help you in your quest to jailbreak your iPhone 3G (first generation and second generation models) and iPod Touch 3.0 with this nifty tool, Redsnow:
Jailbreaking an iPhone 3G (including iPod Touch 3.0) could be tricky so just a friendly reminder: learn how to backup and restore your previous firmware when this update fails on your end.
FonoSIP.com supports Xten / Counterpath SIP softphones
for Windows, Mac, iPhone and Linux.
Internet telephony equipment such as Linksys PAP2, Cisco, Sipura,
Nokia N71. Apple iPhone, iPod Touch, iPad. Android. Codec G729.
Fring Mobile. Bring your Own Device BYOD.
We also support SIP Trunking, replace your phone lines.
Asterisk PBX, TrixBox.