Call forwarding/Tmobile

We are having problems with forwarding calls to a Tmobile cell phone.  Can answer the call but it will drop the call everytme after

12 seconds and last night the call forwarding would not work at all.  I have contacted both Comcast and Tmobile.  On the second call to Comcast I was told that Comcast was experiencing this problem with other Tmobile customers and would call back.  A week later after Comcast not calling back, I called again and was told that there were not open tickets showing there was a problem with Tmobile.  I have contacted Tmobile and they don't have an answer either.  I tried forwarding to a Verizon phone and a US cellular phone and have no problems.  Just wondering if any other customers are experiencing the same problem.