Texting a Mexico number

How do I send a text to a Mexico number? The young lady called my phone from her mobile so I would have the number. It's a Cozumel number and when I text the number I immediately get an error message saying that the number is invalid and to please use a valid 10 digit number. The area code is 987 and when I use just the 10 digit number I still get the error that the number is invalid. Any help would be appreciated in getting this resolved. Thanks!