141 is used to withhold a number. 1470 is used to show a number BUT this may not work for phones within a company which has decided to withhold numbers for outbound calls.
You cannot withold a number when calling from Mobiles - on normal land lines its as well to check with your providor the number to prefix to withhold your number from receivers is 141 or 147 - you can set this also as a permanent feature to withold number from ALL calls.
More useful info on mobile number use on www.mobilenumbers.org.uk
a lot of places that use switchboards wont allow to show number, i have annonomous call reject and when ive had calls from people who use switchboards they have told me they have had to use a private phone or a mobile to contact me,