There is a box for your phone number, and you need to deselect their 'cover' number if you don't want to use it.

Personally I don't use it because it's just too complicated - say you need to phone them back call drops out when asking for directions, then they might not recognise your number and then have it anyway. A lot of guys also are very suspicious of the 070 (or whatever prefix) these numbers carry.

Some ladies have a dedicated mobile for certain adverts, which might be a good idea. I prefer the same phone especially as some TWs will contact you via all your ads, find your number out anyway - and it's useful to be able to recognise them.