Mexican cell phones and sim cards have completely changed the game for travelers. While you can still use your home cell phone (and pay roaming charges, etc), you can also, assuming you have an unlocked phone, purchase and use a Mexican sim card very inexpensively.
Dialing within Mexico, all telephone numbers in the country, whether land line or cell (mobile) numbers now have 10 digits.
If you have an unlocked cell phone, it is easy to purchase a Mexican sim card and use it while you are traveling in Mexico. The simplest way is to find the nearest Oxxo convenience store. There, you can then also purchase the “plan” you need for the time you are in Mexico, or top-up your account at any time. Almost all plans allow you to dial anywhere in North America (Mexico, Canada, US) within the plan’s time or usage limits.
Cell phone usage is now far less expensive in Mexico than (for example) in Canada.