Phone interpreters essentially carry out consecutive interpreting over the phone between one or more parties. This is not to say that simultaneous interpreting can not also be applied in the same way; just that it is used much more in a consecutive manner. An interpreter is usually working remotely, dialed into a conversation and speaks between the parties.
Phone interpreting is commonly used in public services and in areas where physical interpreters are not possible such as at hospitals, for the police and government offices.