This then means using Linksys gear, with SPA2102 or WRP400 the obvious options. I'm aware that either of them supports two FXS ports, and that the VFX provisioning would lock down the 'Line1' configuration. My own Fritz!Box allows me to specify up to ten (or even more) different VoIP accounts. I've also set up two separate ports: one connected to a DECT phone and the other one using the existing in-house cabling, which has up to three corded phones attached. I could choose to let either the first or the second port respond to incoming calls from any given VoIP provider, but to keep things simple I'll just let them all ring for all incoming calls.
After this explanation I now wonder whether my friends can have at least one more VoIP provider (e.g. in the UK) which would also be connected to the one cordless phone that they are currently using. Or do they need to plug in an additional phone (into 'Line2' I guess) because the Linksys gear is only supporting a strict 1:1 relationship between the VoIP accounts and FXS ports? I guess I might be a bit "spoilt" and have too high expectations as my own gear provides all this mini-PBX functionality, but do those Linksys devices support calls between the two FXS ports?
I've tried to search for the answer to these questions in the SPA2102 Admin Guide (and also in this forum here), but I could not get bottom of it. OTOH I'm pretty hopeful that the "usual suspects" can help me straight away.