Only the phone jack conecting to my modem works since changing my phone servies over to comcast. I am upset by this because I can not really get to the phone near it to make calls or check voicemail. I have my phone internet and voice all with comcast now and it is causing more problems then having seprate serviecs. I also bought a cell phone extender from verizon since I do not have service in my house but have tried multiple ways to get it to connect but it still has not worked.
If your house has landline wiring you should be able to disconnect the outside service and jack a line in from the cablemodem to any of the connectors to use your existing phones. But really, the modern way is to buy one of those multi-phone wireless sets, connect the base station to the cablemodem voice jack and put the others anywhere you want in the house. They are not that expensive and very good these days. Probably cheaper than fixing any problem you might have in your old wiring.