Very interesting.

I had a similar problem a few years back with a technophobe Grandma who wouldn't use whatsapp and a stingy aunt who'd moved to the states.

Used a Fritz!Box as it'll act as a DECT<->SIP server so my grandma could keep picking up her usual handset. Had a rule which matched the aunt's phone number so it got bounced via my FreeSwitch server rather than her PSTN, and some least-cost routing rules to pick a US based provider SIP provider with a +01 PSTN number so my aunt saw the incoming call looking local and could ring back if necessary at a cheap rate.

Honestly, the weird technical abominations we come up with to try and serve family members. The same Grandma went through a phase of falling, not being able to get up and then waiting hours for an ambulance to arrive. Would she tell the family? Would she sod. Was her regional ambulance service still dispatching jobs via pager message? You betcha. Did this end up with an SDR, POCSAG decoder and a regx looking for her postcode? Absolutely not; why would you ask such a thing?