Why green bubbles and not blue bubbles on both iPhones (17e) latest iOS

Can the other iPhone send iMessages to other devices successfully?

Did you already disable iMessage, restart the iPhone, and enable iMessage again?

Can you use the email address to send & receive iMessages on this particular iPhone?

The phone number is correctly selected and active for iMessage in Settings/Apps/iMessage/Send & Receive?

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If you experience issues like conversations in Messages showing up as separate threads or sent messages appearing as green message bubbles instead of blue message bubbles when you set up a new device, update your settings using the following steps:

1. Update your device to the latest version of iOS or iPadOS if needed.
2. In the Settings app, tap Cellular. Make sure that your phone line is turned on. If you use multiple SIMs, make sure that the phone number you want to use is selected and turned on.
3. In the Settings app, tap Apps. 
4. Tap Messages, then turn iMessage off and then back on.
5. Tap Send & Receive.
6. Tap the phone number that you want to use with Messages.

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copied from: If you can't send or receive messages on your iPhone or iPad - Apple Support

The reply I received from INGO2711 solved my problem and answered my question. Thank you for your time & consideration!

Glovehawk

You're welcome, glad I could help 🙂