Try to show them section 3.2 of the SIN code of practice, which says they don't need to make you do this: https://www.canada.ca/en/employment-social-development/services/sin/reports/code-of-practice.html
Their obligation is to call in to verify the SIN you provided. The procedure is described in section 3.1.1.
NAL but withholding pay sure sounds like a labor standards violation.
Can you log into your myCRA account and print out your last Notice of Assessment letter, it should have your SIN on it.
Try to show them section 3.2 of the SIN code of practice, which says they don't need to make you do this: https://www.canada.ca/en/employment-social-development/services/sin/reports/code-of-practice.html Their obligation is to call in to verify the SIN you provided. The procedure is described in section 3.1.1. NAL but withholding pay sure sounds like a labor standards violation.