Q: Not able to activate iphone using outside US sim

I got a replacement handset from Apple iphone 6s 9.3.3 and it was working fine in US. During my recent travel to India, I tried to unlock the phone but the phone says Sim card you inserted in not support. I tried to reach apple care and they re-directed to my carrier. Upon discussing the issue my carrier said that this is a non t-mobile phone and we cannot unlock it. I then called back apple and they said they do not usually unlock phones.

Now, I dont know the phone belongs to which carrier and how do I unlock it?

I will appreciate, if some one can help me with this..