I have a MICS 4.1 with T1 PRI's. I had the phone company(not A&AT) to monitor a DID when I call forward it to AT&T cell number. He saw the 10 digits going out and told me that the system was not disconnecting.-- When it is forwarded to a AT&T cell it will give a quick ring(sometimes) and goes into a busy when the DID is called all the time. I can forward it to other numbers(land lines)with the same area codes and it will forward and no busy on DID when called. I have other 4.1 systems and I forwarded to a AT&A cell no problem. They changed providers in March to fix their problem--but no fix-- I have never seen this problem. What can it be????