They are usually VoIP phone providers, they are just running a phone connection over the data you already paid for. Modems and fax machines are pretty sensitiv

They are usually VoIP phone providers, they are just running a phone connection over the data you already paid for. Modems and fax machines are pretty sensitive, and pushing that sound over a packet switched network with UDP instead of a circuit switched network leaves a lot to be desired. So there are some places that still support old copper lines, but in many places, if they have to run a new line, they'll run a coax or fiber to your house and give you a plug in off of that. It's basically not an offering in many places now.