Whenever a customer places an order by calling the call centre for the first time, an User account is automatically created for that customer with the phone number and email id provided by the call centre agent. After this, the customer can login into the app or website using the same phone number or email.
Since no password is provided by the call centre agent when placing an order on behalf of the customer, a random password is assigned by the system when the User account is created. When such call centre customers want to login to the app or the website, they use the "Reset Password" feature to set a password for their account.
Sometimes, for certain reasons, the call centre agent will not be able to get the customer's email, in which case they usually put a dummy email id (like <phonenumber>@<merchantname>.com) to place the order. In such cases if the customer tries to create a new account with the same phone number but different email id, it will result in an error since an User account with the same phone number already exists. In this scenario, the merchant admin or the call centre agent can update the email id.
Details for update the user login are available here : https://prestoapps.freshdesk.com/support/solutions/articles/8000047095-how-can-i-update-the-phone-number-or-email-id-of-a-customer-