iMessage Not Activating on iPhone After Switching Between iCloud Accounts

I was logged into my personal iCloud account with iMessage activated on my phone number. During app testing, I logged out and signed into multiple sandbox accounts I created in App Store Connect. I repeated this process 2–3 times with different sandbox accounts before finishing my testing and logging back into my personal iCloud account.

After returning to my personal iCloud, I noticed iMessage was no longer connected to my phone number. Despite multiple attempts, iMessage fails to activate with my phone number on my iPhone.

Here’s what I’ve tried so far:

• Toggling iMessage and FaceTime off, restarting the phone, and turning them back on.
• Updating to the latest iOS and repeating the toggle/restart process.
• Logging out and back into my iCloud account.
• Switching phone carriers and activating a new eSIM.
• Switching to a physical SIM with a new carrier.

Interestingly, iMessage is successfully connected to my phone number on my new iPad, but it refuses to activate on my iPhone (an older iPhone 11 Pro).

Any advice on how to resolve this?