Apologies for the you experience. Simply dial *123*7*3# or dial *123# (MTN’s self-service USSD dial) and follow the prompt of inputting ‘7’-Self-Service, then ‘3’-Get your PUK code to access the service.
You would be required to answer 3 questions to validate line ownership. When answered correctly, the PUK Code will be displayed on the 3rd party phone and SMS shared also. All inconvenience regretted. ~ Lasbery