Dropped calls to CoxVOIP land phone

I have two T-Mobile phones. I can call my area code 405 Cox home phone from my 404 area code phone and it goes through okay.  However when I call from my area code 770 phone the call gets dropped after a few rings. Both cell phones are located in Oklahoma, area code 405. Cox has checked all equipment at home and everything works.  Calls to the house phone from other cell phones also work okay.  Di I have a problem with my phone or could it be a T-Mobile network issue?  Both phone are Android.