How can I block specific telephone numbers on my iPhone 4?

What Canadian telephone companies block 900 numbers by default?

• I work for a non-profit and we are wondering if 900 numbers (where people call in and they get charged a small amount per call) are accessible by all Canadian land line phone numbers? If not, what providers such as telus etc automatically block calls to 900 numbers?

• Answer:

  Here in the US most providers will block 900 numbers at the customers request. I would guess Canada would be the same.