You can submit a ticket request to ask and see if a moderator can over-ride this feature.
What happens is if your phone number is from a specific city from your previous provider, and the current provider doesn't have anymore of those phone numbers, you will run into a problem when you try to port your phone number over.
For example, phone numbers in Richmond, BC no longer are actively given out for some of the Telecommunication companies in Canada. If a user wishes to port over from Company A to B but his original phone number was from Richmond, BC, sometimes they'll run into porting issues. Some companies are able to bypass this using an internal system when they port phone numbers, or they can contact their porting centre for more help.
In your specific case, if Public Mobile cannot override this at this time, you won't be able to bring over your phone number unfortunately.
