Also works with 7941G. These phones are very different than the 79x0 series, but I have one successfully working with CallWithUs. Support instructed to use server carrier.callwithus.com. I have the following ports forwarded to the phone's internal IP address: UDP/5060, UDP/16384-16393

Replace the following stings in the file:
%INTERNET_HOSTNAME_OR_IP% (can use dynamic DNS or public IP address for NAT router on user side) %SHORTNAME% (label appears next to line button on phone's display)
%CWS_ACCOUNT_NUM% (CallWithUs username, note that this appears 3 times for each configured line - name, authName, and contact elements)
%CWS_ACCOUNT_PASSWORD% (CallWithUs account password)

The file name must be of format "SEP" + phone MAC address in hex + ".cnf.xml" and be ANSI/ASCII (not UTF) encoded.

The following entries on the VOIP Wiki are also good resources for configuring Cisco 79x1 phones without using the Cisco CallManager PBX: http://www.voip-info.org/wiki/view/Asterisk+phone+cisco+79x1+xml+configuration+files+for+SIP http://www.voip-info.org/wiki/index.php?page=Standalone%20Cisco%207941/7961%20without%20a%20local%20PBX