Pots plain old telephone service Sip session initiation protocol (basically a form of voice over ip)

Pots still exists so op answer isn't correct. Phone companies prefer you switch to digital because when calling on their network there's no handoff and fees to other carriers.

On pots say you live in Arizona and have century link. You call your mom in Denver who also has century link. There's a lot of distance in between and the call is going to pass through several networks owned by other companies. Century link has to pay a fee everytime a call travels out of their network. If you and your mom have voip the call stays on century link network since it isn't passing through analog switches so no hand off fees.