Sky's help guide here says:
"The diverted part of the call is charged to the Sky Talk customer at the standard Sky Talk rate depending on the type of phone the call is diverted to, e.g. mobile. The person calling is charged for a standard UK landline call and will be charged as normal by their calls provider."
My uinderstanding (and I might be wrong) of that is that you pay for the diverted part of the call, i.e. a call from your landline to your mobile.
