OnSIP customers dialing certain Canadian cell phone numbers get busy signals. These cell phone numbers are provided by Rogers Communications and one of their subsidiary companies, FIDO. The busy signal only affects callers who are not sending a caller ID. Make sure your account has a valid caller ID associated with it. In OnSIP, this can be accomplished via the web portal under Users  (for individuals) or under PBX Settings- Default Caller ID (for the entire company).