@BarinderJassal
It is important to remember that it may not be the original owner of the phone that has blacklisted it but the "owner" of the account that the phone was originally acitivated with a sim card for service. As in a couple with the account in the husbands name but with the wife having a phone and plan under the same account. Technically the husband can blacklist the phone even if the wife bought the phone from a seperate retailer and has the reciepts and credit card charge to prove she bought it.
As mentioned a provider can blacklist a phone that has not had the device subsidy fully paid off. When the device lands in the hands of a third party like you they have even refused to allow payment of the debt owing in order to remove it from the blacklist and transfer full ownership to you. Success in this regard is very YMMV.
To find the original provider look for any account apps that may be emeddded in the phone or possibly the other APN for providers that appear alongside public mobiles. There may be provider logos near the IMEI, behind the battery if it is removable or in the settings under "About phone."
A phone can also be blacklisted by a device insurance company when a claim is made and paid out. If the claim is substantial enough they may do this to ensure that a repairable phone that costs more to fix but still works is not used when they are paying out a replacement cost. Or they will list it if its lost, stolen or they determine an insurance fraud has taken place. This could account for a long period of time passing between you purchasing the phone and it only now be blacklisted as the claim and all appeals may have taken this long to be settled. You could check with the main couple of provider device insurance companies that exist in Canada.
Otherwise for these claims do have to be reported to police if a device is lost/stolen so there is a possibility that if you filed a freedom of information request that you could find out who filed the police report for the device with your IMEI.
Saying all this your phone is at least 3 years old. While perfectly functional at that age maybe its best that in your mind that you consider your phone lost/stolen and buy a new device. You could still sell it to a buyer outside of Canada to recover some of its value.
