For BMO Mosaic Mastercard Support, here are the numbers to call for help, as from the back of a BMO Mosaic US Dollar Mastercard, which seems to be the only no-fee US$ card in Canada:

USA/Canada: 1-800-263-2263

Overseas (Which you may call collect): 514-877-0330

Here's a list of telephone numbers to call most of the Canadian Mastercard Issuer's Customer Support Numbers.

Call these numbers regarding disputes, lowering your interest rate, cancelling your card and more:

Canadian Tire Mastercard 1-800-459-6415
MBNA Canada Mastercard 1-888-876-6262 
HSBC Mastercard 1-866-406-4722
BMO Mosaik Mastercard 1-800-263-2263
Citi Mastercard 1-800-387-1616
CapitalOne Mastercard 1-800-481-3239