It's mostly this https://en.wikipedia.org/wiki/G.711 https://en.wikipedia.org/wiki/G.711 nowadays. Real POTS(landline) or ISDN is exceedingly rare. There are

It's mostly this https://en.wikipedia.org/wiki/G.711 https://en.wikipedia.org/wiki/G.711 nowadays. Real POTS(landline) or ISDN is exceedingly rare. There are some other standards, but they have to be supported on both sides of the line, which is not a given, especially not between different TELCOs/ISPs. Which makes trying to use FAX over it really 'interesting'. See https://en.wikipedia.org/wiki/T.38 https://en.wikipedia.org/wiki/T.38 Technically one should be able to run something like V92/Flex over it, when the stars are aligned right. But nobody does that anymore, because why?