Thank you for the details of the issue you are experiencing with iMessage. I recommend the steps below, but steps 3-4 in the second section seems most applicable to the situation you described:
To resolve issues with sending and receiving iMessages, follow these steps first
- Check iMessage system status for current service issues.
- Go to Settings > Messages > Send & Receive and make sure that you have registered iMessage with your phone number or Apple ID and that you have selected iMessage for use. Learn more abouttroubleshooting iMessage registration if the phone number or Apple ID isn't available for use.
- Open Safari and navigate to www.apple.com to verify data connectivity. Learn more about troubleshootingcellular data or a Wi-Fi connection if a data connection isn't available.
- iMessage over cellular data might not be available while on a call. Only 3G and faster GSM networks support simultaneous data and voice calls. Learn more about which network your phone supports. Go to Settings > Wi-Fi and turn Wi-Fi on to use iMessage while on a call if your network doesn't support simultaneous data and voice calls.
- Restart your device.
- Tap Settings > General > Reset > Reset Network Settings on your iPhone.
If you are still unable to send or receive an iMessage, follow these steps
- Make sure that the contact trying to message you isn't blocked in Settings > Messages > Blocked.
- Verify that the contact you are attempting to send a message to is registered with iMessage.
- If the issue occurs with a specific contact or contacts, back up orforward important messages and delete your current messaging threads with the contact. Create a new message to the contact and try again.
- If the issue occurs with a specific contact or contacts, delete and re-create the contact in the Contacts application. Create a new message to the newly created contact and try again.
- Back up and restore your device as new.
Thank you for using Apple Support Communities.