DMZ shouldn't really be used, though for brief periods of testing it will probably be fine. Just remember to remove your device from DMZ when you're done.
Are you using Google Voice or a SIP service provider? If SIP, it's possible that a SIP ALG on your friend's Arris modem is causing the problem. If there is no setting to disable this, try changing your X_UserAgentPort to a random number between 20000 and 65535, and also changing your ProxyServerPort to some other port number supported by your ITSP (if they listen on another port number).