I have made the mistake of using 141 to temporarily withhold my number.. The call goes through fine, but your number isn’t actually withheld..
Giffgaff, please disable further use of 141 and don’t let calls go through. I have called many a business and only today did I accept my number wasn’t actually withheld. This was realised when one cheeky company called me back "offering" me more rubbish.
For your information, i am now using #31# before the number, which is the correct way to withhold your number on the Giffgaff network.