If you have a BT line, your calls to mobiles are costing you 12p/minute during her week and 3p/minute at weekends with 18866.
You could simply change to 1899 for those mobile calls and cut the cost to 10p/minute during the week (they also now charge 3p/minute at weekends).
That's about 17% cheaper during the week.
The downside is 1899 make a 3p connection charge as opposed to 18866's 2p.
It is possible to get cheaper rates at some times to some networks (e.g.
OneTel's Standard UK Talk and
OneTel's CallSaver packages would save money on some calls) but you'd have to be very aware of the networks you're dialling and the relevant times to benefit.