How to verify an Apple employee call about iPhone

Did you call them and request a call back?

What did they want to know about your phone?

You can verify with Apple directly by calling them. Contact Apple Support - Apple Support

If you did not solicit information for Apple to call you back, I see no reason why that would be the case. I can say my employee number is 1234 and that really does not prove anything.

See this:

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

The only unsolicited phone calls I ever received from Apple were when a device was nearly at the end of its one-year complimentary warranty. They were calling to sell AppleCare. That has not happened in 10-15 years, and we buy all our Apple gear direclty from Apple.

Such messages today come to my email address associated with my Apple ID. Their email domains always end in "@apple.com" not @apple.something.com.

Called about what exactly? Was this in response to something you initiated?