Sounds like you've been shifted from POTS ("Plain Old Telephone Service", analog, AM, voltage down a wire...) to VoIP ("Voice Over IP"; encoded signal stuffed into packets and transmitted over the packet network before being reassembled at the receiving end).

The wall jacks won't work -- no encoding, that lives in the modem -- but presumptively an old-school wireless phone meant to work with a wall jack would to provide multiple handsets.