A free lite version of VoIP engine for devices which runs Windows Mobile has been released by Global IP Sound. As per the company:

The GIPS VoiceEngine Mobile Lite platform includes a collection of components from the core GIPS technology along with sample integration code for quickly building and testing VoIP applications.

The company claims that its technology is being applied by a number of VoIP application providers such as Inter-tel, Tencent and Skype. Besides this, the VoiceEngine Mobile Lite is interoperable with Google Talk's peer to peer and voice calling capabilities.

As of now it is supporting Wi-Fi enabled devices which run Windows Mobile 5.0 and Windows Mobile 2003. You can avail it for free by registering at the GIPS Developer Community. The community includes demo applications, software and an online forum for peer to peer development support. Besides this you can also access VoIP test tools and video tutorials.