If you have the older version PA-2A with only five screw terminals, your PA-2A is designed to detect ring signal that was “earth ground referenced” and you lost this earth ground reference when you changed to another carrier.  Depending on the PA-2A you have, you may be able to get the loud ringing to work on one line.  Contact technical support for assistance in doing this or if you need the product manual for the version of PA-2A you have.  If you need multiple lines to loud ring, you will need to purchase a new PA-2A.  For more information related to this problem, see DOD # 906.