Third Party VoIP Over Xfinity

[Matt Hoppes <mattlists () rivervalleyinternet net>]

I have an employee who has recently switched to Xfinity cable service. 
Ever since they switched their internet service their work phones will 
not stay registered for more than about 3 minutes.

These same phones have been used on many ISPs without issues.  The same 
config has been used behind multiple levels of NAT without issues.

She was fine, until she switched to XFinity.

Of course, XFinity support is absolutely worthless.

Anyone from XFinity Tier 3 or such that might be able to offer assistance?

I suspect it's something stupid with either NAT overload in the modem or 
the modem not keeping the SIP channels open.

I've tried playing around with registration times without any success. 
And again, we've never had issues with these phones or this setup with 
any other ISP.