How to fix iMessage activation error after changing eSIM or country SIM?

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.

1. iMessage by Default: If both you and the recipient have iMessage enabled and both devices are connected to the internet (Wi-Fi or cellular data), your messages will be sent as iMessages (blue bubbles).
2. "Send as Text Message" Option: If the recipient’s device is not connected to the internet or their iMessage service is temporarily unavailable, the message may not go through. When that happens, you get an option to "Send as Text Message" — which sends the message as a regular SMS (green bubble).
3. One-Time SMS, Not Permanent: Choosing "Send as Text Message" only applies to that specific message — not future ones. Once both devices are back online and iMessage is working, future messages will continue to be sent as iMessages automatically.
4. When SMS Becomes Default: If iMessage remains unavailable for the recipient for an extended period, you can go to Settings > Messages and toggle "Send as SMS" — this lets your phone fall back on SMS when iMessage isn’t working. But again, it won’t override iMessage when both devices are connected to the internet.

See the pic below