OK - very good. Now that none of the home phone are connected and you STILL hear only ONE RING in the cell phone, we now KNOW for sure - that the Panasonic system is NOT the cause of the issue.
Now from the symptoms you describe now, it would be my diagnosis that this is a PROVIDER issue. That is, the service provider (you mentioned Frontier) is not allowing the calls to ring more than that one (or 1.5) times. This sometimes happens when a "call block" feature has been turned on, but that would have to be verified with the service provider.
I have the main contact number for Frontier below, and recommend that you contact them as soon as you can to report this issue.
Frontier Phone - Technical Support & Repair
Contact Center Hours of Operation: 24/7
Please let me know what is found or if you have any more questions.
Thanks, ***** *****
P.S. Now that we have determined that the Panasonic system is NOT the cause, you can reconnect the main base unit to power and the phone line, just like when we started.