Because they need a data link to their own equipment in the exchange.
Thanks, understand now. Guess it will depend on demand to see if it's worth it for them.
Not bothered about changing to BT as such but expect TT would be a cheaper option.
I am on an old TT offering and get free phone calls to many international places. Also get caller display for free which I understand BT is now charging for.
All such things need to be taken into account.