Can't iMessage certain iPhone users

[Speedy]

A month ago my son, my wife and I were out of the country. My daughter stayed home. All with iPhone 5's. The three of us who were away did not have cell service, only WiFi. My wife and I could text with our daughter but my son couldn't. Nor could he text with us. We tried to match every Setting on his 5 to be the same as ours but to no avail. We gave up and used Viber. Sometimes it just doesn't work. Like it was a Microsoft product.