If you're trying to sign up for a Zip account and get an error message about your email address or mobile phone number already being in use, it means that you already have an account in the system linked to your details. It may be that you signed up with Zip a while back but don't remember the number or email used. Prior to November 2019, ZIP was known as PartPay, so you could have created a PartPay account which has been automatically converted to a Zip account.