The most common routers here in Germany have POTS connectors and convert that to VOIP internally. At least for POTS with DTMF. I keep one specific router around that even still supports pulse dialing as I have two rotary phones from the 1950s and 1970s still hooked up for fun. (They work, except for the fact that nobody knows the number of my land line)
When the biggest provider in Germany switched from POTS and ISDN to VOIP there was also a number of solutions available for bigger installations, so you could keep your POTS installation while the adapter does the VOIP conversion.
So all in all switching worked mostly fine.