IPhone 5s blacklisted

I’m After to some help. I have an iphone 5s which has just sat in a drawer when I upgraded to the 6s but want to use it for gym with a spare SIM card.

The phone was purchased by myself from Apple on day of release.

I’ve inserted the sim and get no signal, a lot of diagnosing and researching online I have found it to be blacklisted.

I spoke to my network/carrier who also have confirmed then imei has been blacklisted but unable to tell me whom has applied this. They have also provided me written confirmation that neither I have requested nor have they applied the blacklist.

Although it’s an old phone it’s still my property and want it to work as intended I paid around £700 for this. Whom can tell me who has blacklisted the phone so that I can get it removed?