Not quite what you are looking for but my ex landline phone is plugged into my LTE router currently fitted with a Voxi sim, supposedly 8gb for £10. Last month I used 105gb without using any of my 8gb. Not too sure how long it will last but anecdotally has been going on for some 4-5 months.
I've heard of VOIP, but I want a phone I can plug into the back of my wifi router and get free calls.
Is that possible?