Cordless Phones losing charge after switch to Comcast

I was not present at my elderly mom's house when the technician made the switch from Verizon to Comcast (landline) phone and internet service (due to DSL being the only Verizon service available at her location and it being abysmally slow). Since the switch, the phones, which had no previous problems and according to my mom are plugged in as always, have been displaying low battery and one saying no power to base. Is there any reason that a change to Comcast voice would have resulted in something going on with charging the phones? TIA.