The times have really changed. Remember having to use a fax modem to dial to get on the internet using a analog/pots (plain old telephone service) line? Well now in most places you have to have internet service to get telephone service. Majority of telephone service providers are no longer providing or servicing analog telephone lines, but have switched to VoIP, using fiber or coax to bring internet to the premises and then converting it to a digital telephone line.
All of this magic is accomplished by something called SIP (Session Initiation Protocol). SIP creates a standard and defines how telephony and video communication devices communicate with each other. Allowing different vendors that makes this equipment to speak a standard language that all devices can understand. Now of course its not that simple but once again we’ll spare you all the long boring stuff.