You soon will have the opportunity to "Unmask" withheld calls as your phone is still ringing.by pressing a key two times, and within 3 seconds the "Withheld" will be replaced by the caller`s number, an exception being ,if the caller is known to you and is in your contacts list , up will come the name that you know him by ie John B /work. The service initially will be open to mobile phone users who are on "Contract", and giffgaff PAYgo customers. it is thought that other PAYgo service providers will also join the scheme too, and most likely will charge 1 minutes call time allowance for each "Withheld" unmasked. Look out for WithheldNumbersUnmasked.co.uk and YourNUMBERsUp.co.uk... for more info on the latter, do a Google search (Withheld Number Unmasked Giffgaff) , i think its about the 3rd one down the page of results.
You can only call back a withheld number from your cell phone by first revealing the number itself. The only way to do it would be calling up your network carrier to tell you the withheld number. However, you may not be able to get this information without a legal order.Learn more tips on how to unmask a withheld number at http://www.ehow.com/how_6760329_out-number-withheld-call.html.