Older PA-2A units (with five screw terminals) detect the ring signal that is "earth ground referenced". (from legacy copper CO phone lines)
Moving to another source of dial tone likely brought about a change in the ring signal - hence our unit is no longer able to properly detect ringing.
Depending on the PA-2A you have, it may be possible 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 the need is to have loud ringing for multiple lines, it will be necessary to purchase a new PA-2A.
For more information related to this problem, see DOD # 906.