SMS not received


I have used the SMS action for a few years now, but this is the first time I have been faced with this issue. I am sending a SMS to a Mexico phone number and the person is not getting the message at all. I have included 10 people in the distribution of the message and the other 9 people are getting it, but just one person isn't. Has anyone faced this issue? if so, how can we solve it? He is using an iPhone, not sure if this helps.