Incorrect Phone Number Associated with Your Apple Account

It sounds like a perfect use case for Apple Business/School Manager's Managed Apple ID feature. With a Managed Apple ID, you can reset the phone number or password for the user.

Pair Apple Business/School with a Mobile Device Management (MDM) platform, and you can unlock devices that are managed by your org.

Hello ErikCarlson,

Thanks for choosing the Apple Support Communities. It sounds like you have two issues here. Changing phone numbers associated with Apple IDs and getting Two-Factor Authentication codes for your devices. Let's address each.

1. The first thing we'd recommend is setting up a trusted contact for the devices. That way when they change phone numbers or need an authentication code but don't have access to their phone number, they can have it sent to that trusted contact and get a code. Here is how: Manage two-factor authentication for your Apple ID from iPhone - Apple Support

"Add or remove a trusted phone number

When you enrolled in two-factor authentication, you had to verify one trusted phone number. You should also consider adding other phone numbers you can access, such as a home phone, or a number used by a family member or close friend.

1. Go to Settings  > [your name] > Password & Security.
2. Tap Edit (above the list of trusted phone numbers), then do one of the following:

• Add a number: Tap Add a Trusted Phone Number.
• Remove a number: Tap  next to the phone number.

Trusted phone numbers don’t automatically receive verification codes. If you can’t access any trusted devices when setting up a new device for two-factor authentication, tap “Didn’t get a verification code?” on the new device, then choose one of your trusted phone numbers to receive the verification code."

2. You can then change the number associated with the Apple ID, and have a trusted number to get an authentication code: Use your mobile phone number as your Apple ID - Apple Support

"On the web

1. Sign out of every Apple service and device that uses your Apple ID.
2. Go to appleid.apple.com and sign in.
3. In the Sign-In and Security section, click Apple ID.
4. Enter the mobile phone number that you want to use as your Apple ID, then select Change Apple ID. 
5. A verification code is sent to you at that number. Enter the verification code from the text into the fields provided.
6. Sign back in to all Apple services with your new Apple ID.

On your iPhone, iPad, or iPod touch

1. Sign out of every Apple service and device that uses your Apple ID, except the one you're using to change your Apple ID.
2. Go to Settings > [your name] > Name, Phone Numbers, Email. 
3. Next to Reachable At, tap Edit. 
4. Tap Delete  next to your phone number and follow the instructions.
5. After you enter the mobile phone number that you want to use as your Apple ID, a verification code is sent to you at that number. Enter the verification code from the text into the fields provided.
6. Sign back in to all Apple services with your new Apple ID."

We hope this helps.

Cheers!