There is unfortunately no easy fix for the issue as the slot does appear locked. You the fore will have to contact Cellc's customer service centre and epress you wish to use other SIMs in the slot in question. They will the quote you the subsidy they paid which you will have to pay to them. Alternative to that, you can avail the services of the Volcano Tool Box if you can find any cellphone repairs shop with it. The latter may be cheaper but harder to locate any that own the box in question.