Well, if you can spare a line button you can build a service that does just that. There are no XML commands for it, so you either have to use (J)TAPI or AXL. There's a service for callforwards at myvoipsource.org that may serve as a template. I'm afraid it is not possible to add buttons to a template that execute a functionality not available for normal templates.. so it has to be a service on a line button (of which you have two on the 7940.. a 14 button add-on sure comes in handy in such a situation).