The scope of ED-137 is to define the rules for VoIP implementations to support ATM communications and their interoperability. ED-137 is composed of multiple Volumes, each of which is dedicated to a specific application. Volume 2 proposes a profile standard for the use of SIP to establish, terminate and modify speech media sessions of the Ground Telephone Service in an Air Traffic Services Ground Voice Network (AGVN). Volume 2 is composed of a core document describing the basic requirements and a number of addenda dedicated to specifc functionalities. Addendum 3 describes the requirements for Instantaneous Access Calls.