For decades, most telephone service was done over copper wire, using analog. Then came digital telephone sets/systems. However, there were limitations. Mainly, that copper wire can only handle so much voice or data (internet traffic).
More telephone and internet providers are replacing that old copper infrastructure with Fiber Optic cabling. Fiber is far better for today’s needs.
VoiP telephones have all but replaced the old analog and digital systems. Few if any manufacturers of telephones are making the old systems anymore. Further, fewer are making new parts for them. As those older phone systems begin to break down there is a greater likelihood that there will not be a fix available anymore so moving to VoiP is inevitable.