The OP clear implication is that their relative is paying too much for the benefit they receive by having broadband and phone , and asking would removing broadband be possible (it is but not simple) but the fact is the reduction in price by removing broadband to leave just a phone service would be relatively modest, and present practical issues and the obvious thing to do is get the same deal as most every other BT broadband customer gets , this would be a substantial reduction in their bill ( likely more than the savings paying for a standalone phone service while still having broadband )
Franky the suggestion to get a second broadband service , then port the number to cease the original broadband and then use this presumably cheaper broadband with a paid for VoIP provider after purchasing the ATA equipment is nonsensical especially when considering the OP title is ‘what happens to phone service if I cancel broadband’
