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US WIRELESS PHONES IN MEXICO
AT&T Discounts
AT&T
Since they use GSM technology, roaming is extensive throughout Mexico, some of it on AT&T's newly upgraded 4G network. They now offer  "AT&T Mexico Roaming Bonus" to Mobile Share Value consumer and business customers for free which allows unlimited texting to anywhere in the world and unlimited voice calling within Mexico and from Mexico to the United States. Additionally, customers will get 1 GB of data to use while in Mexico (with overage charges of $20 per GB).

Cricket
CRICKET WIRELESS
Cricket is owned by AT&T. Cricket phones will work in Mexico for Talk, Text and Data at no additional charge with the $50 and $60 plans and currently uses a mix of AT&T and non-AT&T networks.
 



MetroPCS
METROPCS
MetroPCS is operated by T-Mobile. New GSM MetroPCS phones roam in Mexico with an add-on that gives you the same Voice, Text, and Data allowance while Roaming in Mexico as you have in the US. Limited to plans $40 and over. Depending on when you add the feature, the "Mexico Unlimited" plan is either Free or $5/month. Old CDMA MetroPCS phones don't roam outside the US.
Sprint
SPRINT
"Normal" Sprint phones use the smaller CDMA network in Mexico. Under Sprint's new "Open World" plan, you can roam in Mexico and several other countries across the Americas with no additional charges for Voice, Text and Data. You will need a Sprint  "International" phone and there are a few limits such as roaming percentages and 1Gb of Data.
T-Mobile
T-MOBILE
T-Mobile phones work well in Mexico since they use GSM technology, and reports are they use multiple networks. T-Mobile's regular plans allow you to roam in Mexico with either reasonable, or no, additional fees. Their "Mobile without Borders" plan allows you to roam in Mexico (and Canada) with no additional charges for Voice, Text or Data. This includes calls TO & FROM the US, Mexico and Canada with Data included, with download speeds up to 4G LTE rates, if you can find it.  T-Mobile Pay-As-You-Go phones will also roam in Mexico but the per-minute rate is very high. T-Mobile's "Wi-Fi Calling" app allows voice calls made through wi-fi at no additional charge but in light of their broad roaming capabilities, wi-fi calling is a better option for where you have no cellular signal but do have wi-fi.



U.S. Cellular
US CELLULAR
US Cellular phones with postpaid plans roam in Mexico, although the roaming networks are not as extensive as GSM networks. Rates include a $3 per day surcharge, plus $.99 per minute, plus any pass-through toll charges from the foreign carrier. Text and Data roaming are not currently available.
Verizon Wireless
VERIZON WIRELESS
Older Verizon phones use the smaller Mexican CDMA network, which serves most larger cities and resorts. If your Verizon phone finds a signal, it will work. Newer Verizon phones include GSM circuitry, so they will roam on one of the better GSM networks. They offer Mexico Minutes Plans which allow 1,000 minutes of roaming in Mexico for about $15 more than regular plans. Data roaming charges are $.005/KB ($5.12/MB). Plans without free Mexico Roaming, incur a charge of .99 per minute for both incoming and outgoing calls within Mexico. Text Messaging in Mexico is .50 Sent, .20 Received. Both postpaid and prepaid Verizon phones work in Mexico at the same per-use rates. Verizon offers a Global Data Roaming option. While it's not cheap it could prevent you from getting surprisingly high foreign data roaming charges.

Prepaid
PREPAID PHONES
There are several prepaid phones and plans that will work in Mexico, some are preferred over phones from the above Postpaid carriers.