iPhone blocked to a carrier in Chile

How to unlock an iPhone to be used with international carriers?

I bought an unlocked iPhone in Chile, to which I've used the last year. Now that I am traveling I want to used it with carriers from Colombia. When I try to use another SIM card, a message appear with this "Sim not supported". After this, I tried to contact my carrier in Chile, and they said they could not unlock my phone and to contact technical support to unlock my phone. I appreciate your help.