Transitioning Devices: Backup Verification and Phone Number Removal from Apple ID

Dear Apple Community,

I’ve been using an iPhone 15 Pro Max for about a year. The device was purchased by my employer under their name, but I have been paying all the bills and managing everything related to it. Recently, I purchased an iPhone 16 Pro Max under my own name.

I have a few questions and would appreciate your guidance:

1. When I backed up my old iPhone to iCloud, I excluded many files and apps because the cellular backup process was taking too long. Now that I’ve already started using my new iPhone, how can I ensure everything from the old device has been fully backed up and transferred—especially the content I initially excluded—before erasing the old phone?

1. This is the more important question: Since the phone number will be returned to my employer, how can I make sure it is completely removed from the old device? I understand I need to update my number on all websites and apps, but what should I do specifically within Apple’s system to ensure the old number is no longer associated with my devices or Apple ID?

Thank you so kindly.