Are there no landline style options these days you can shove a SIM in, or maybe even a little box you can dummy out to a landline style phone with said same? There are a few for fallback options and mobile phone companies hate them for reasons I have never quite figured out (I can understand the landline bridge stuff but the other way around seems strange).
Don't know what I am suggesting for private VOIP these days, and it is generally super rare in the UK (give or take most phone companies being VOIP companies but different discussion there) for domestic setups. Most things I do on that front are complicated setups for companies that care more about a good setup (multiple lines, being able to speak to a robot despite being a local restaurant, times in which you can forward onto the boss, dial in answer phone and such fun) than the baseline cost.
Also are you really tied to unlimited minutes? So many claim to want that but reality says otherwise, even more so if you put your foot down, get phones from everybody and install a decent text/video/audio chat program such that everybody can go every which way and have group chat (anybody need something from the shop type deal). There is also the unlimited minutes but does not include mobile phone problem.
I imagine you would have mentioned if you were particularly wedded to a number but it is something to consider in this.
Do also check if you are doing the granny still has an "I am surprised this is not a rotary phone" routine if said 80s phone is pulse dialling or tone dialling. Many won't support pulse dialling. You may also want to grab some RJ45 cables as some of these adapters might not support the BT style phone connector.
Found it 
