Comment by ralferoo

I'm not sure if they're still available, but the PAP2T was a wonderful piece of kit - just an ethernet port and 2x RJ-45 jacks for old style phones. You can set up custom dial plans and use any VOIP SIP provider you like. You can trivially edit the dial plan to restrict certain number prefixes, and set up custom short numbers - so for instance my "landline" had 81 for my mum, 82 for brother, 83 for sister, etc. but you could also just dial regular numbers.

If you run an asterisk server on your own box, you could easily set up a private SIP network just for you and your kids, or your kids and their friends, etc. and either run a SIP client on your mobile for your use and a VOIP SIP gateway if you want your kid to be able to call a friend's mobile.

EDIT: I just looked and the PAP2T has been discontinued, but there seem to be lots of units available new from China that look identical and are sold as Linksys PAP2T, and some unbranded units that look the same but with blank labels. I've no idea if these are fully compatible with the real PAP2T, but they might still be worth trying.