A business requires a GST number if it sells taxable goods or services. A GST number becomes mandatory once your revenues exceed $30,000. However, if your revenues are less than $30,000, obtaining a GST number is voluntary. You can register for your GST (Business Number - BN) by calling: Canada Customs and Revenue Agency Phone #: 1-800-959-5525. Please Note: When registering for a GST number by phone or in person, you must ensure you have a social insurance number (SIN).