In regular use, you enter a code from an app, rather than using any network features of the phone. You can also set up (either as default or as backup) a phone

In regular use, you enter a code from an app, rather than using any network features of the phone. You can also set up (either as default or as backup) a phone number to text with a code -- as backup, they recommend sending to someone else's phone. Lastly, they will give you a list of ten one-time-use codes which you can write down or print out and put in your wallet/safe. If all the above fail, I believe there's an account recovery procedure, which takes a couple of days and involves sending proof of ID. So you're not relying on a single device, and certainly not on a single phone number, to be able to retain your account.