Had the exact same problem today with my BYOD line, TF calls dropping after 30 seconds. Opened a ticket with support and had calls re-routed for specific TF numbers I use daily, but any other TF numbers still have this problem. If you are able to look in you logs you will see that all TF calls routed through 207.223.64.xxx have the problem. This is a new route I have not seen before today, but clearly there is something wrong with this route.

It would be nice if they could take this route out for all TF calls until it is fixed, but the only thing you can do is ask that your specific TF numbers be re-routed due to this problem.

Good luck.