10 miles clear line of sight is no problem at all, assuming the aerial the other end is a high gain sector, correctly elevation aligned and more importantly, facing your direction.
They can use a higher gain aerial at your property if needed, however these are more expensive than the standard small dish / integrated units .
If they have engineered it properly you will not regret moving away from the BT line. Ask if they support VoIP too, if they have done it right then the answer will be yes.
Then you can throw away the ~£16 BT line rental, say hello to genuine cheap calls and say goodbye to connection/min call charges, inflexible features and your number being sold on to marketing companies repeatedly!
We use www.voipfone.co.uk
as do many of our home and business customers and recommend them highly.