As of Aug 3, 2019, the rules of dialing a Mexican phone have changed:

To call a Mexican cell phone and land line:  +52 (phone number including area code)

The ‘1’, ’44’ or ’45’ is no longer required.  The ‘+’ automatically directs your call to an international switch-board-computer.  You must dial a + for all international calls.

To call a Mexican cell phone:  +52 1 (phone number including area code)

To call a Mexican land line:  +52 (phone number including area code)