I'd like to thank you for coming to us with this question, as I'm sure others will be thinking the same. The answer would be that it does all depend on the Talk Plan that you subscribe to. You pay a monthly charge for the Call Divert feature, this is £2.25 a month, however if you have two or more features on your land line, then it will be £3.90. Check out our available Call Features on this handy link, please not some features may be unavailable in some areas.
Once you've paid the monthly fee, then the diverted calls will be treated the same as if you were to make a call from your land line. In your case, unless you're on the Talk Unlimited Extra package (which includes calls to all mobile networks) you'd be charged the same to divert the call as it would to call your mobile.
For people who wish to divert calls to another line (a land line or Virgin Media Mobile) then they will find that the charges to Divert a call would be zero during the time that their Talk Plan is in action, out side of the talk plan then regular call charges apply.