Why can a phone number only be associated with one AppleID?

Are you trying to add it as a trusted number for 2 factor authentication or are you trying to add it your own Apple ID?

Adding it as an actual trusted number should not elicit the warning as you aren't actually tying it to your Apple ID. Any number capable of receiving a voice call or SMS can be added as a Trusted phone number, and I don't believe there is any limitation if it has already been added as a trusted phone number for another Apple ID.

click here ➜ Two-factor authentication for Apple Account - Apple Support

Also, you should not ever add your own iPhone's phone number as a trusted phone number as that as you said defeats its purpose entirely.

Apple does not say anything in its documentation about numbers being limited, but I guess we must be in the era where everybody in the world has their own telephone number, right? (Gosh, I remember when we had a single telephone and 6 in the family).

You're trying to add a second number as outlined in this article? Given that everybody in the world supposedly could have an Apple account, and everybody in the world has their own telephone, then in theory it would be impossible to have a second trusted number if what you're saying is correct.

Two-factor authentication for Apple Account - Apple Support

Then I guess you must have a unique number and in the World According To Apple everybody has their own telephone number. Do you have to have a telephone number in there? Apple lets you use another Apple device as a trusted way to get a code so in theory you don't even need a number. I don't know if they will let you set it up without one though. Never tried.

As far as your scenario concerns about a stolen device, if a device is stolen your should mark it as lost (Find your lost iPhone or iPad - Apple Support). Even if somebody has your passcode, Apple has recently introduced steps making it difficult for a thief to change settings if you can act promptly: About Stolen Device Protection for iPhone - Apple Support

boudewijnk wrote:

I want to protect my AppleID with a phone number.

Even if my iPhone (+passcode) would be stolen, I still want my AppleID to be secure. In this situation, my phone number being a trusted number doesn't make it more secure, because someone else would receive the sms messages.

I’d expect Apple wants better security there, but wants to make having a zillion Apple Accounts a little more difficult.

Consider a recovery contact: Help a friend or family member as their account recovery contact - Apple Support

That allows a way to regain access without allowing an easy path to take-over.

Additional details to consider:

To prevent a stolen iPhone with a compromised passcode: About Stolen Device Protection for iPhone - Apple Support

Convert SIM to eSIM, to prevent SIM swap: About eSIM on iPhone - Apple Support

Or maybe set a SIM PIN, but that's much more work: Use a SIM PIN for your iPhone or iPad - Apple Support

Enable two-factor: Two-factor authentication for Apple Account - Apple Support

If you’ve got the budget for a few, switch to security keys: About Security Keys for Apple Account - Apple Support

Related: Better Securing Your Data, and Apple Acco… - Apple Community

boudewijnk wrote:

Thanks for the many suggestions.

I use two factor authentication and I have a recovery contact. I also have stolen device protection on Always (even when I am at familiar locations), so I always have a one hour delay before certain things can be changed. However, once that one-hour-period ends, a recovery contact can easily be deleted.

If you can’t get to a place where you can dis-trust the disappeared iPhone within the hour, and if passcode exposure is a risk for you, then it’s either longer passcodes or passwords, or you’re prolly then headed for security keys.

I’m using long passwords on iPhone (usually 12 to 15 characters), and using Face ID, and I’m not too concerned about somebody surfing that password.

If you have an Apple Watch, this (I’ve not tried it) might work: https://www.tomsguide.com/how-to/how-to-lock-your-iphone-from-your-apple-watch-with-a-focus-modes-hack

I’ll probably log an enhancement suggestion for this later today, too; a supported way to remote-lock the paired (and stolen) iPhone from Apple Watch.