Must I set APN?

Hi, I bought the Tether for Blackberry sometime ago and recently I switched my mobile provider to M1 from Singapore. I keep getting the 'Failed to find available port' error as my APN setting is empty. I called up the provider and they refused to reveal the APN for Blackberry Internet Service. The APN in the (http://www.webmessenger.com/support/APN.jsp) is not under the BIS hence the data is chargeable. Could you advise what other APN I could use, for example Blackberry.net, or Is there anyway I could find out the APN. Thanks in advance.