Phone use in Cancun

Well you didn't say what plan you were on because unlimited is not a name of a plan. But it's likely you have talk and text in and between the USA and Mexico.

If you log into your account online, and go to your customer service summary page it shows all of your included features. It may include North American Roaming which means you have both Canada and Mexico talk text and data included. 

If you log into your account online, and go to your customer service summary page it shows all of your included features. It may include North American Roaming which means you have both Canada and Mexico talk text and data included. 

Just be aware that not seeing that or something similar does not mean Mexico is not covered. Unless  you have a very old unlimited plan, it probably does include Mexico. I have a grandfathered plan and my plan features don’t list anything about North America or Canada or Mexico roaming yet my plan does include Canada and Mexico. If you tell us the name of your plan, we can verify.