If you've recently changed OS types (Apple, Android, Windows, BlackBerry) or are unable to send/receive text messages on your iPhone or messages from other iPhones aren't blue (iMessages), this may help.
The steps below apply to the following models:
iPhone X / XS / XS Max / XR
iPhone 8 / 8 Plus
iPhone 7 / 7 Plus
iPhone 6 / 6 Plus / 6s / 6s Plus
iPhone 5 / 5c / 5s
iPhone 4 / 4s
From the Home screen, navigate: Settings > Messages .