As already stated , it’s the way the system ( imposed by Ofcom ) works , if someone either mistakenly, by giving your fathers address rather than their own , or maliciously , by slamming your father service onto another company without consent , the only safeguard is a communication ( letter or email ) from the losing company ( BT in this case) saying ‘sorry to see you leave ‘ ,but it does give the process to cancel the migration, if that goes unanswered then the changeover takes place…it isn’t a two stage process , where the customer has to sign up with a new provider and also advise the company they are leaving themselves , the regulator doesn’t want the losing company the opportunity to try and talk the consumer out of switching.
If your father lives in some sort of warden controlled housing , it’s possible a ‘neighbour’ changing provider, or a new neighbour moving in , has given the wrong address ( your fathers address ) , unfortunately the losing company are not allowed to ring and ask ‘are you sure’ ?, the letter or email should arrive 10 days before the date the changeover is to take place.
TBH , if this is migration your father didn’t ask for , although the BT service will have been removed , it should have been replaced with the new company service ( even if it’s not what he wanted ) ,so there still should be service from someone, if there is no service whatsoever, that may not be an (unrequited ) migration but some other ‘issue’
