Yes it will work.
Last 4 month I was traveling every week there to Mexico City and Toluca.
There are 2 GSM carriers that have agreements with AT&T:
Movistar and Telcel.
I found Telcel to have better coverage.
The roaming charges are 1$/min, if you buy the Mexican package for $5/month you lower the rate to 0.6$/min, so it makes sense that you activate that service for just 1 month and the get rid of it when you come back.
The data roaming will be very expensive, but it works if you need it.
Use the turn off data roaming setting in your iPhone to disable it.
Where are you going?
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