The Samsung A20 international version phone I purchased on Amazon and later added a T Mobile sim card did not work very well here in Maine. Locally, US Cellular owns many towers and T Mobile customers can use these towers with most phones. This international version was not on the list of approved devices so I could only use T Mobile towers - which are pretty consistent along interstate highways but pretty rare elsewhere in Maine. Then there are the bands each phone works on... It's complicated. So I purchased the same phone - but not the international version and it works well. Time (about 2 months) will tell how it works in Mexico.
T Mobile covers calls and texting in the US, Canada and Mexico.