Features - Call Restrict
 

The outbound Call Restriction service allows you to block calls to expensive or undesirable destinations, e.g. premium rate. This is intended primarily for our outbound call services such as VoIP or FNP.

You can even choose between blocking specific numbers and blocking a whole number range, such as all international 00 numbers or premium rate 09 numbers, so you don't have an unexectedly high bill

Just logon to your account, and click Call Restriction. Now type in the number you want to restrict and add a label to remind you about that destination. Then press the Restrict a Destination button. You can apply restrictions to just one number or for 'All Numbers' contained in your account.