From a historical standpoint, AT&T once held a minority stake in what is now Rogers Wireless, hence why it had been known at various points as "Cantel AT&T", "Rogers Cantel AT&T", and "Rogers AT&T Wireless". There is no longer any such connection.
From a functional (and also historical) standpoint, Bell Canada would be the closest equivalent to AT&T in Canada, especially since Bell Canada was once considered to be part of the Bell System as a Regional Operating Company. It was only a partial ownership (peaked at 49% in 1885), but both AT&T and Bell Canada spawned from Alexander Graham Bell's original telephone patents (the US and Canadian versions of the patents respectively).
