We have had this service for many years with bt, it means we can always receive office calls whether we are at home or on the mobile.
You just phone up bt and ask for the redirection service to be enabled on your line, there is a small charge for the service like there is for caller display, bt are trying to reduce charges now though to compete with rival providers so it might even be free now.
Anyway they will ask you what landline number you want to enable the redirection service on, and you can ask when it will be enabled from - as in when will you be able to use your redirection service from. Sometimes they can enable it there and then on the phone, other times it can take 24hrs to enable.
The redirection service allows you to divert landline calls to other landlines or to mobiles, by picking up your landline phone pressing for a dial tone then press *21* (then the number you want to divert to) #. The voice will then say "your calls are now dirverted to " they they repeat back to you the number you are dirverting to" then just hang up. To cancel the diversion pick up the phone get a dial tone and then press #21# " and then the voice says diversion service cancelled"
Cost for the calls are that the person calling you pays as usual to call you landline but you pay for the cost of the call from your landline to where ever you are. Therefore calls to a dirveted mobile cost more. If you are out of the country that could be expensive.
But to be honest as it is work and I have had no troubles with the bt diverstion service ever, and I would recommend them over anyone else.
I have heard they also offer a remote diversion service now. At present if we divert to the mobile and then want to divert to our home landline we have to go back in to the office to use the office phone and landline to re-divert. With the remote system you can change the number you divert to from the