The easiest thing to do is not worry about your phone number and use other resources available from whoever runs the account. Usually, you can use email accounts, backup phone numbers, security questions, and other things to prove yourself. If you absolutely need the old phone number, contact your carrier for help.
The +1 in front of a number refers to the country code of the dialer. Every country has a country code (plenty of countries share codes, like the United States and Canada). When someone calls you, you can see their entire phone number, and that will include the code for the country they are calling from.
This is mostly done to get around software that is looking at phone numbers on the website.
Craigslist sometimes uses software that blocks or removes phone numbers from listings for a variety of reasons. There are also third-party bots that crawl the site looking for phone numbers and contact information.
To remove a number, go to your contact on the phone and select the option to delete the contact. This will remove your number from the phone, and shy of any backups, is a permanent deletion of your number. If you can’t access the phone, you can block the other person’s number or explore other alternative actions.
Phone companies and carriers own phone numbers. They are in charge of deactivating and reactivating them, along with recycling them. If you have an old phone number that you want back, contact your carrier. If your carrier can’t help, you might have to negotiate with the new person who has your old number.
There are a few reasons why Uber says your phone number is not supported. Potentially, your phone number is already in use, or it is an invalid number. Or sometimes, it is because you are unable to get through to its 2-step verification process, which of course results in being unable to sign into Uber.