On August 3 the Mexican phone system will remove the 044, 045 and 01 prefixed now in use. This applies to all phones, Mexican and foreign.
Beginning 3 August 2019 all in-country calls to and from landlines and to and from cell phones will require dialing only a 10-digit phone number; that is, the area code plus 7-digit phone number.
Calling Mexican toll-free (800) and premium toll (900) numbers. When calling from a Mexican land line or from a Mexican cell phone, drop the 01. So 01-800-123-4567 becomes 800-123-4567.
For emergencies, continue to use 911.
— Mexico’s Federal Institute of Telecommunications http://www.ift.org.mx/comunicacion-y-medios/marcacion-a-10-digitos and other sources