Why won't my iMessage work with only one contact on my iPhone ?

iMessage (Blue) -

• iPhone to iPhone: requires internet (Either over WiFi or Cellular Service) as it is sent over internet protocol. Needs Activation on your device.
• iPhone to Android: Not available

SMS/MMS (Green) -

• iPhone to Android: does not require any internet it is sent over cellular voice protocol. Needs to be enabled by your Carrier for the SIM used by you.
• iPhone to iPhone: When iMessage is not activated or the device is not connected to the internet either over cellular or WiFi
• You need to have an SMS/MMS plan enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc. does not provide SMS/MMS services.

It will turn from Blue to Green if you have enabled "Send as SMS" and whenever your iPhone is not connected to the internet it is sent as SMS (green) to all iPhone users.

There are a few reasons why your texts might not be delivered to your friend's iPhone even though you can receive theirs:

iMessage Issues:

• iMessage Not Enabled: Both you and your friend need to have iMessage turned on for texts to exchange as iMessage (blue bubbles). Check-in Settings > Messages > iMessage.
• Data Connection: iMessages rely on data or Wi-Fi. Make sure you both have a stable internet connection.

It's possible that you used their Phone Number to send an iMessage, while they might have sent an iMessage using your Apple ID. There's a chance that iMessage isn't activated for their phone number on their device.

iMessage requires phone number activation for sending and receiving messages with other phone numbers. Without activation, you can only chat with people using their Apple IDs.

Activate iMessage and FaceTime using your… - Apple Community

It sounds frustrating that you can send messages to everyone except one person! Let's figure out what might be happening:

1. If the recipient has an iPhone:
   1. Double-check the recipient's contact information: Ensure you have the correct Apple ID or phone number entered. Sometimes, a typo or outdated information can cause glitches.
2. Verify iMessage settings:
   1. 1. Check if iMessage is enabled on your iPhone. Go to Settings > Messages and ensure "iMessage" is toggled on.
3. If not ON, try sending messages using your Apple ID to the recipient.
4. To activate iMessage and FaceTime using your Phone number
   1. 1. First, check to know whether you have an SMS plan enabled for at least One SMS*** with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you
      2. If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One international SMS*** Only)
      3. If you can't turn on or sign in to iMessage or FaceTime - Apple ...
5. *** You may not have an SMS plan (Domestic & International) enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc. does not provide SMS services.
6. Ask the recipient if they have issues receiving messages: Maybe they're experiencing problems on their end.
7. If the recipient has an Android phone:
   1. Ensure your phone plan with your carrier includes SMS messaging. Without it, you won't be able to send text messages to non-iPhone users.
   2. While iMessage does require the internet, SMS doesn't. 
   3. Check your network connection to ensure you have a stable cellular connection. Poor connectivity may affect your ability to send SMS.
   4. Reach out to the person you're unable to message and inquire if they are experiencing any issues with their Android device or network connectivity.

Swipe left on the conversation and you’ll see the option to unmute it. I just found it myself, after trying every solution I found online. 🤦‍♀️

If you are still not able to send, then...

1. Is this your corporate/school iPhone? Then please contact the system administrator of your corporate/school
2. If it is your personal device, in that case, do you have any profile installed by your corporate/organisation/enterprise/School Management?
   1. Erase the profile added by you If you have installed a profile on your own --> Install or remove configuration profiles on iPhone - Apple ...
   2. Or please contact the system administrator of the organisation who provided the profile to you