Do you ?!? I've diverted my office landline to my mobile a few times, when I've been working on my own, and it's never crossed my mind that I'd be charged !!
My company recently saved a client several 1,000 pounds a year in just this area. He was regularly out of office and since he was a sole trader he routinely had his office phone on divert to his mobile. I was able to source him an excellent alternative fixed line and mobile tariff, slashing his overall telecoms bills by around 50%.
Until we sat down with him and analysed his bills, I dont think he had realised that the call diverts comprised around 40% of his bill!