I have one POTS line I am using a CISCO SPA-3102 ADA to interface with 3CX pbx. I have a ring group assigned to answer the line and everything works fine unless no one in the ring group answers the line. If that happens it should forward to another backup ring group. When the transfer tries to take place the caller receives a fast busy signal. I have setup my VOIP lines to forward in this manner and the caller never receives a busy signal. Any idea what would cause this to go to a busy signal going toward the secondary ring group?