Two things that seem to help whenever I'm dealing with bizarre Comcast issues....have her call in and:
* Ask for "Security Edge" to be disabled if it's enabled (last time we did this Comcast told us they couldn't permanently disable it unless we paid a lot more per month for service and it would automatically enable every reboot, but another rep permanently disabled it for us)
* Ask them to disable "Smart Packet Detection" if she's using a router that has that feature
Those two features seem to mess with a lot of traffic--specifically DNS (re-routing any unencrypted DNS request to Comcast's own servers) and SIP.
Of course enabling TLS for your SIP connections would probably help significantly--not just with connectivity, but security.
-A