Have you recently taken over this line, or have you had this line for a long time, with no changes and the problem has only just started ?
If you have recently taken over the line, try dialing 1280 in front of the numbers you are having trouble with, if you get through OK , perhaps the previous user of the line had CPS (carrier pre select) where although they paid BT for line rental, the calls they made were billed separately by the CPS provider ( this was a thing years ago, but a lot less common these days)
If the line has been in your name for a long time with no changes and the problem has just started, then you need to confirm if it's your phone at fault, you could try borrowing another phone and trying that in the master socket test port ( not via a broadband filter ) or take your phone and trying it in another known phone line ( maybe a friend,neighbour or relatives line )...if the phone turns out to be OK, you may have a line or exchange problem, if the phone is faulty obviously you will need to replace it
