For years I had DSL, with one PSTN line and one VoIP line, all distributed through house wiring. The trick was to put a filter on the incoming phone line at the NID, then split it, with one pair going to the PSTN half of the jacks (inside pair), and the other pair going to the VoIP adapter, and from the VoIP adapter to the outside pair of the jacks. It worked fine, but the inside wiring needs to be pretty clean. Mine runs in a star pattern out from the phone closet.