Make Sure Send & Receive iMessage Is Turned On

1. Open the Settings app.
2. Tap Messages.
3. Scroll down and tap Send & Receive.
4. Look for a check next to your phone number under “You can be reached by iMessage at” and “Start new conversations from”.
5. If there isn’t a check mark next to your phone number, tap it to give your iPhone the ability to send and receive iMessages.

Why this works:

Turning on Send & Receive gives your iPhone the ability to send and receive iMessages from other iOS devices. If send and/or receive is turned off, your iPhone won’t be able to send or receive iMessages in the Messages app.