A verified telephone number does two things: it lets Quavern reach you about the account, and it can stand in as the second step at sign-in while nothing stronger is set up.
Set it up in Security on my.quavern.com.
Adding the number
Enter the number starting with the country code, as in
+33 6 12 34 56 12. It must be a mobile line, because the code arrives by SMS.Confirm your current password.
Enter the six-digit code that arrives.
Turning SMS into the sign-in step
Once the number is verified, switch on "Ask me for a code by SMS when I sign in". Turning it on gives you ten one-time recovery codes, shown once. Keep them: a code by SMS cannot always be sent.
This switch is only available while the account has neither an authenticator app nor a passkey. If it has one, the number stays a contact and the switch says so.
Why adding a passkey demotes it
Adding an authenticator app or a passkey sets the SMS factor off in the same transaction. Your phone stops being a key as soon as either exists.
An SMS code can never disable, replace or bypass a stronger factor. If it could, every account with an authenticator app would still be only as strong as its mobile number — and a mobile number can be taken over without touching your devices. So SMS is a fallback, never an alternative route.
The number itself stays on the account, and stays verified. It goes on appearing masked, last two digits only, wherever it is shown.
Changing or removing it
Changing the number asks for your current password, then a code sent to the new one. The old number stays the factor until the new one answers.
Removing it asks for your password. It stops being a way to sign in, and Quavern can no longer reach you on it.
One number, two accounts
A single number can verify at most two accounts. Beyond that it is refused. Two is what a household, or one person with a personal and a company account, actually needs.
If you cannot receive a code
See "Verifying a telephone number and reading the code".
Quavern help reference: telephone-second-factor/en