I tried to activate FaceTime and iMessage on my iPhone 15 Pro after update to iOS 18. But i could not activate it to my service provider phone number and its activated to my iCloud mail id. Also tried everything mention on Apple Support page. No luck yet. I contact my service provider too. They asked to clerify from Apple.
Our engineering teams need to investigate this issue, as resolution may involve changes to Apple's software. I'd greatly appreciate it if you could open a bug report, include an some directions for reproducing the problem, and post the FB number here once you do.
Bug Reporting: How and Why? has tips on creating your bug report.