Intra-business calls are the ones you make between your own fixed, BT Mobile, and VoIP lines. There are no call charges for the first 60 minutes of these intra-business calls. For calls that last longer than an hour, the extra minutes are charged at the pence-per-minute rate.
No more than 75% of your total minutes can be intra-business. If you exceed this, BT reserves the right to terminate your BT Complete contract.