iMessage failed to activate

Good morning, everyone. I've been using iMessage and FaceTime for two years now on my iPhone 11 Pro. One day, after receiving my device back from an Apple certified service center where I had a battery replacement done, my phone encountered an issue with verifying my phone number for iMessage activation. I reside in Greece, and my mobile carrier is COSMOTE Group, with a contract that fully supports all iMessaging features.

Here's the problem:

• When I navigate to Settings > Messages and attempt to enable iMessage, I encounter an initial message that states, 'Your carrier may charge for SMS messages used to activate FaceTime and iMessage.' After clicking 'OK,' I proceed to select 'Send & Receive.' I have two email addresses connected to my iCloud and Gmail, both of which are selected. However, my phone number remains stuck at 'Verifying...' with the only available option being 'Remove.'

Upon restarting the process and removing my phone number, I find myself caught in a loop with a persistent and buggy pop-up message that continues to state, 'Your carrier may charge for SMS messages used to activate FaceTime and iMessage.' After clicking 'OK,' I receive an error message that reads, 'An error occurred. iMessage cannot be activated.'

Furthermore, when I navigate to Settings > Profile > Name, Phone Numbers, Email, and look under the "EMAIL & PHONE NUMBERS" section, I can see my phone number. However, above it, there's a message that states, "Unavailable for iMessage and FaceTime." Upon clicking the information sticker, I'm presented with the message, "This phone number cannot be used by iMessage and FaceTime," along with the explanation, "You must have a SIM associated with an Apple device for +(my number) to be used for iMessage and FaceTime."

After an entire week of persistent efforts, which involved reaching out to both Apple support and my mobile carrier while diligently following all their recommended steps, I've now reached a point where I'm actively engaged with an Apple technician. This technician is currently in the process of analyzing my device's data and has installed a profile onto my phone to investigate the underlying issues.

However, I remain unable to use my email addresses for iMessage. This is a matter of utmost importance to me due to privacy and security concerns, as I prefer to keep my email addresses confidential. If anyone can offer any potential solutions or advice, I would greatly appreciate your input. I will compile all the steps I've taken thus far and include them at the end of this article. I will also provide updates if my issue gets resolved in the future. Your assistance is invaluable in this matter.

This is what I've tried so far :

1. Cleared the SIM tray.
2. Restarted the device.
3. Backed up the device.
4. Cycled Airplane mode on and off.
5. Stayed connected to Wi-Fi for 48 hours continuously, keeping the iMessage phone number verification menu open.
6. Changed my email password.
7. Ensured correct input of my phone number in both Phone Settings and Profile settings.
8. Removed my phone number and re-entered it.
9. Performed a force restart of the iPhone.
10. Verified that the device is running the latest iOS update, version 16.6.1.
11. Checked the ability to send and receive SMS messages (with more than 2000 SMS remaining in my mobile carrier app).
12. Reset network settings.
13. Left iMessage and FaceTime turned off for over 24 hours.