how to advise id moblie of my pac number

Hello @Nji603015,

https://my.idmobile.co.uk/web/cpw/pre-registration/-/registration/registerMyAccount to register for an iD account. (If you've already registered, you can skip this step).
To register you'll need to put the iD SIM in the phone, enter your last name, date of birth and the iD number currently active (not the one you want to keep). A text with a security code is sent to the SIM, then once you receive it, enter it in the registration and setup a password.

Now log-into the iD app/account
Click " My Services "
Click "Keep your number"

Enter the PAC code and number to keep
Click Submit.

Mohammed