Actually are work being reasonable?

[Humuhumunukunukuapuaa]

In asking me to divert my phone line to my own personal mobile when I'm working from home?

Who was behind you working from home? If its a cost-saving iniative then they should cover all the costs properly. If however its because you wanted more flexible working them I think you would be unwise to demand they gave you a phone too.

Buy a cheap PAYG so you can switch it off though. And keep track of your calls: surely you can just submit a list of calls/length/cost/customer at the end of the month?