Unlock iPhone 6 plus

I Have an iPhone 6 plus 16gb whick is currently locked with my old network provider. I already finoshed my contract with them but they can not process the unlock of my device since my account with them is already closed. They said that they can only unlock my iphone if I have an active account with them. Is there any way I can use my iPhone to a different network provider or any other way to unlock my iPhone?

Thanks!