Ok, tech support's change didn't help.

However, I switched the customer over to the new Chicago server and everything started working much better right away.

Incidentally, I've noticed that most (but not all) of the clients having trouble are using Comcast Business Class for internet service.

In the past, I've had trouble between Comcast users and the Texas server. I moved a ton of clients over to the Michigan server (sip7) to resolve. I had started moving them back to Texas when Voipo announced they were dropping the Michigan server. I think some of these issues have now returned. So I'll likely have to start moving people away from the Texas server again.