 The functional service responsibilities of an RFSS in a call are dependent upon the type of call and also on the role of the RFSS in a call.The specifications developed in the ISSI Messages and Procedures for Voice Services standard provide the fundamental starting point for the remaining standards in the ISSI suite of standards.The ISSI Measurement Methods for Voice Services is based on the functional services and protocols defined in the ISSI Messages and Procedures for Voice Services standard.