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 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: