It’s important to maintain control of this phone number. For example, you could use a disposable SMS service to register with Signal — there are many such services if you search for them — but those phone numbers can be used by anyone. Similarly, you should avoid using a public payphone’s number, or a SIM card on which you do not intend to renew service. If someone else can receive SMS messages or phone calls to this phone number, they can take your Signal account away from you.
That risk is not just theoretical. I made a test account (on another service; not Signal) using a free anonymous SMS number. A few months later, the account had been hijacked.
Of course, if it's a disposable account, then having it hijacked after you're done with it might be a good thing.
Register a new account over that phone number.
They can't get into any previous accounts register with that phone number.
They could potentially manage to find the pin if the previous user really used a guessable one, but then again, they won't be able to check the previous messages and the linked owner of that account will be warned of that new connection.
If someone trys to register with an existing number then it wont work if its already being used.
Im not sure on this^
Better to enable a security pin if you are concerned.
The traditional phone system involvement is annoying.