We have a similar problem! We set it up to my mobile when we were planning to go away in case of any urgent medical calls. Now we are home we can't remove it. We have tried the #21# and doing it on the app. also tried changing it to the calls not answered in 15 seconds. It doesn't work from the phone and from the app we get an error message! This leaves me with a big problem in that if I am out and need to contact my husband the call will just be diverted back to me!
My husband is now getting very frustrated spending hours on hold to BT!
I moved your post to start your own thread as the previous thread was marked solved and this way you will replies to your problem
you can try message now and if they cannot help hopefully they can direct you to someone that can
https://www.bt.com/help/contact-bt/account-and-billing/broadband
Thank you.
My husband did manage to get through to the tech support this afternoon and they do seem to have turned it off for us now. I don't think we will try using it again, maybe just putting my mobile number on the Voicemail message would be safer!