I am not sure if this helps, here goes;
land line (BT) (call diversion is about £5 a quarter I think)
0800 (Utility Warehouse) £4.11 per month.
So I pay BT for the fixed line and divert facility. The 0800 number is attributed to the land line and I pay additional monies for calls made through the 0800 number.
The divert can go to any phone I choose - which is invariably my mobile.
Regards
Mark