In my experience BT call divert is very expensive. I use a call divert thingy(!) that can divert a call to any number -you text it what ever number you want it to divert to. Works by using a mobile sim, just get a cheap 1 month contract sim with how ever many mins you need. Went from spending £100plus per month to £20 for the sim
EDIT its a stand alone device that your regular line is conected to, and diverts as and when is necessary