Fring now support windows mobile.
Here is the description from fring’s website:
fring™ is a free mobile VoIP software that lets you talk and live chat (IM) using your handset’s Internet connection instead of costly mobile airtime (GSM) minutes. fring enables free mobile calls over Wi-Fi Internet access or your GPRS, EDGE or 3G Internet data plan.
fring lets you easily communicate with all your fring, Skype®, MSN® Messenger, Google Talk™ , Twitter and regular phone contacts from one, integrated contact list. fring also supports SkypeOut and hundreds of SIP-based services such as GizmoProject, VoipCheap, VoipStunt, Free World, even from non-SIP enabled handsets, enabling cheap local and international VoIP calls to contacts’ PCs, mobile phones and landline numbers. fring is also enhanced with real-time contact availability (presence) so you can see who’s available before dialing.
Here is a post about fring.
It looks like fring’s service is unstable. But at least, fring could be used as a free sip-client for mobile. I’ll see how it goes.