The phone can only be serviced in the U.S.
It is illegal to export the iPhone to Iran or to sell it to anyone who has the intention of taking or shipping it to Iran.
If the phone needs to be serviced, you will have to physically bring it to the U.S. and take it to an Apple Store for service. Apple will not accept international shipments.
You could send it to someone you know in the U.S. to take it in for service, but it is against the law for them to ship it back to you.