What is done with/happens to an iPhone if taken as evidence?

There is no set rule as to what will happen with the phone. It will be kept until the case is concluded. Once the case is concluded, depending on how it was used as evidence, it will be returned to the owner. In other cases, such as where the phone contained child pornography, the government might want the phone forfeited. The disposition of the phone is usually worked out as part of the plea agreement.