Many years ago I was a telephone designer. I'm afraid it was too long ago for me to claim to be an "expert" on modern equipment, however. But here's something to consider:

People say that you need to plug a filter into each socket which serves a 'phone, modem or fax machine. But many people see this as a license to plug as many devices into the filter as they can find adapters for. Now, it seems to me that a filter's crossover point may be affected by the impedance of the connected equipment. If I'm right, it may be a bad idea to fit a filter then plug more than one phone/modem into that filter. Instead, you would need to fit a filter to each individual phone/modem and connect the filters to one or more sockets. Am I making sense?

Top diagram shows several devices plugged into a multi-socket adapter which is plugged into the filter.

Bottom diagram shows several devices, each plugged into its OWN filter, which is plugged into a multi-pin adapter.