I just had another customer call for support this morning.

I had switched them from SIP7 to SIP during the SIP7 outage.

Not long after the switch, they started having trouble with busy signals on outbound calls, and also trouble getting voicemail.

I switched them back to SIP7 and all problems solved.

They are using Comcast for internet.

There definitely seems to be some issue between some Comcast users (and some other cable users) and the SIP server, but SIP7 works for them.