The normal way of call barring is to have a default outgoing route and then lock down undesired numbers. When locking down un-desired numbers it is important to take in to account IP Office dialling rules and add an N after any dial string you are trying to block.

For example to block calls to Premium rate numbers (1900-xxx-xxxxx US or 09… UK):

It is important to ensure that the Telephone Number is followed by an N so that it matches even when dialled en-bloc (or redial).

Many countries have prefixes that may be dialed before normal PSTN numbers, for example to force Caller ID presentation, (*67(US)/141(UK) to Withhold Caller ID, *82(US)/1470(UK) to present Caller ID) it is important to include versions of all barred short codes including these prefixes or just bar any call attempts using these prefixes.