iMessage not activating on my iPhone

I had the same issue and Apple support couldn't provide a solution for over 1 week, for an iPhone SE with standard SIM card.

Then I came across the Apple community discussion (see link below) by searching for "AppleID imessage SIM card".

https://discussions.apple.com/thread/255144885?sortBy=best

Specifically, my issue was that on the [Settings > #YourName# "Apple ID, iCloud, ..." > "Login and Security"] I could see my AppleID and my phone number, but below the phone number the message "This phone number cannot be used in iMessage and FaceTime...you need a SIM card associated with an Apple device" appeared.

Reading through and following approximately the notes of user manishr15, I did the following and the issue was solved for me:

• Turn off iMessage from [Settings > Messages > iMessage]

• Turn off FaceTime from [Settings > FaceTime]

• Turn off wifi and turn on the mobile network (4G or 5G)

• Remove SIM and re-insert SIM. Check that the 4G or 5G mobile network works normally again.

I then did these 3 extra steps just in case, which I thought could be useful - but perhaps not necessary:

•• Went to [Settings > #YourName# "Apple ID, iCloud, ..." > "Login and Security"] and removed my phone number, then added the same phone number again at the same place. (Note: Make sure that you can still receive the security code via SMS or through another trusted device.)

•• Shutdown the iPhone and turn it back on.

•• Checked again that the 4G or 5G mobile network works normally.

So, at this point resumed the notes of user manishr15

• Go to [Settings > Messages] and turn on iMessages.

• Then below on "Send and Receive", I pressed on my AppleID as was shown, and then selected "Use Other AppleID".

This last selection -which seemed rather unreasonable and is not mentioned elsewhere- then restored normal connection to iMessage, OK!

Hope this helps.

To activate iMessage and FaceTime using your Phone number

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 ...

*** 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.

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.

It's possible that you used your Apple ID to send the iMessage, while the recipient might have replied using your phone number. There's a chance that iMessage isn't activated for your phone number on your device.

Activate iMessage and FaceTime using your… - Apple Community

I have an issue with my iMessage