John Rumm wrote:

The router could then be configured to relay incoming calls to
a POTS number of your choice. So in effect you place a phone call that
originates on the PSTN (or mobile networks), gets shifted onto VoIP by
the SIP provider of choice, the router then received that "call" and is
programmed to relay to another POTS line

I like it, so you could make your chargeable calls from home to your 2nd
home, which would in turn force the 2nd home to make its chargeable
calls to some fixed number, killing two birds with one stone ...