IMEI is used to check if a phone is lost or blacklisted.
Did you switch from a previous provider ? If so then you would need to contact them to unlock your phone.
How to unlock public mobile imei?
@busy5000 Public mobile does not lock phones so I'm not sure what your are asking. Your phone might be locked to the previous provider. You must get the Network Unlock PUK code from your previous provider.
If your phone is a Samsung you might be able to get an unlock code from the Samsung website via a chat
To start a live chat session from Samsung's Contact Us page: http://www.samsung.com/ca/info/contactus.html?CID=AFL-hq-mul-0813-11000170.
Hope this helps!