Who knows, is possible to change the firmware of the 7931G phone for using as a SIP klient?
For asterisk, you can try the sccp or skinny plugin, look on voip-info.org.
You will find however that a 1751-v or 1760 does a better and more economical IP PBX, expecially for SCCP phones.
Here is a little added info to Paolo's good post;
Q. What protocols will be supported?
A. The Cisco Unified IP Phone 7931G will support only SCCP at FCS.
Q. Will Session Initiation Protocol (SIP) be supported in the future? When?
A. Support for SIP Version 2 will be available in the Cisco Unified CallManager Version 6.0 release timeframe.
I thought it interesting to find this as well??
Cisco Unified CallManager Express Quick Reference Card for Cisco Unified IP Phone 7931G (SIP)
Hope this helps!
No, Cisco has stated that the 7931 does not supports SIP and there are no plans to do that.
If you want an equivalent phone for SIP, look at Linksys Business One.
Hope this helps, please rate post if it does!