Is it possible to wire a VoIP adapter to all the landlines in the house? If I disconnect the wires that lead into the house from the POTS terminal outside, they will have no input. Then can I connect the output jack on the back of my VoIP router to any landline jack in my house to have VoIP to every landline jack?
Look at the REN (ringer equivalence number) on each of the phones in your house and then look at the specification of how many REN your VoIP ATA can drive. If the number on your ATA is larger than the sum of the REN on all your phones you can do it. (I am assuming your are in North America where the REN is required to be marked on the phones. I don’t know if that is true elsewhere.)