Modem is not good enough, you need an ATA (which can be part of a modem). This is a little device for exampla a SPA-3102 converts the "0110010010100010101" to "Hello MUM".
So digital to analogue (assuming your regular phoneline is analogue) and back.
You do not need a VoIP provider if you want to have all you calls going via the PSTN (POTS). You can use 3cx to dial direct to the PSTN (if you have an ata).
I am still trying to get that back to work so I can report this back in an other discussion on this forum. I had it working but for the live of it I can not get it working again :-(.
Henk.