The assistant is confusing you. It is attempting to register you with
a SIP provider and create a new account at the provider. Does it not
tell you which provider it is suggesting you use? You already have an
account with that provider. Deleting the local config does not delete
your account at the provider. You do not need to create a new account
at the provider. Just tell the assistant that you already have an
account, and tell the assistant your id and password. If you lost the
password, go to the website of the provider, and they will likely offer
to email a reset to you.
For more specific help, you need to mention:
a) what provider you are trying to use
b) what version of linphone (esp mobile vs desktop)
In the linphone program, I launch the assistant and click create
user. After attempting to register a phone number, a message appears
that the phone number is being used, although the account has
previously been deleted. When you try to enter this entry using the
same number, a message appears that the entry does not exist.