Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I would say, quite simply and from a simple viewpoint, if the problem came on with the changing out of the phone line modem, then that's probably what is at fault.
But, what sort of modem, and where in the system, was this phone line modem, please ?
Is it a DSL modem? or some other kind?
I cannot advise you as to a Panasonic PBX box. I have Opted Out, with apologies, in favor of such an Expert.