They may want to withhold their number because:

Most calls from people who have withheld their numbers have no malicious intent. Is it a malicious or nuisance call if it's withheld? If you want to permanently withhold your number, press 9 and call us free on 0800 800 152 or contact your BT Account Manager. Dial 1470 and then dial the extension number you want to call, or 9 followed by the phone number of an outside line.Dial *147# and then dial the extension number you want to call, or 9 followed by the phone number of an outside line.If you've asked us to automatically withhold your number, here's how to release your number: Press 9 and dial the phone number of the person you want to call.Is your number ex-directory, or you don't want to release your number under any circumstances? To permanently withhold your number, head to our sales pages to get in touch with us, and we can arrange this for you.

If you've got a geographic number for the line, you can either display this or the 056 number. Just dial 1470 followed by the number you want to call. If you've chosen to have your number permanently withheld, you can use 1470 to release and display your number on a call-by-call basis. If your number is withheld the message 'withheld' is returned if the person being called uses 1471 to find out the identity of the caller. Just dial 141 followed by the number you want to call. If you're not withholding your number permanently, you can use 141 to withhold your number on a call-by-call basis. These features also let you find out a call back number or find out who called you last. You can choose to hide caller ID (withhold your number) by using 141 or show caller ID by using 1470 when you're making outgoing calls.

How to find out a no caller id number