I just spoke to BT and they said I shouldn't be on PAYG as well as the 500 plan. The two charged calls were PAYG because the 500 plan was being ignored although I was still being charged for it. They have recontracted me for the 700 plan and dropped the PAYG. It's lucky I spotted the problem as soon as I did. Thanks for your help.
There are some calls for which I prefer to use the landline, but yes I do use the mobile for others free.