You can normally type 141 in front of the number you are calling to withold the number.
If you are ex-TW you can find a guide for you region below :
http://help2.virginmedia.com/help/ge...age=help_phone
<SNIP>
Number Conceal
Allows you to keep your phone number private – it won’t be presented to the caller you are calling.
Using Number Conceal - Just dial 141 before you make the call.
Number Conceal Permanent
Lets you keep your number private every time you make a call.
Using Number Conceal Permanent
If you want to leave your number, dial 1470 before you call. If you are ex-directory,
you’ll still need to ask for Number Conceal Permanent. Just call Customer Care
to set it up.
</SNIP>