US SIMple Calling Plus Service Mexico carrier?

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If you live in, or are a regular visitor to the USA, Canada or Mexico, the SIMple Calling Plus service is for you. You get a United States phone number and enjoy a combined T-Mobile and AT&T coverage (the largest GSM network operators in the USA), as well as coverage in Mexico with IUSAcell and 3G coverage in Canada with Bell and Telus.

Who is IUSAcell?