Depending upon your network you may be able to do it via dialing a prefix in front of the number, e.g. #31# on Three.
Even though most WGs say they will not answer calls from with-held numbers, in practice they do as they don't want to miss out on any potential business. However, I would still recommend getting a punting phone as opposed to withholding the number of your regular phone for a number of reasons. One of which is that it's handy to get a text from the WG with her location - e.g. postcode & flat number. If you have to rely on getting this over the phone, most likely in broken English then you're more likely to end up at the wrong door.