Following is a list of the most common supplementary services: Resume Music on Hold Blind Transfer Attended Transfer Three-Party Conferencing 10 SIP Trunking Call Forward on Busy Call Forward All Each of these supplementary services requires a complex call flow, and as such, this capa- bility of the PSTN and TDM network can cause a great deal of interworking issues when transitioned to SIP trunks. The most basic capability of HOLD and RESUME in SIP requires changing the media parameters of a call. The standard method for changing the media parameters for a call is to use a REINVITE that changes the call state from SEND RECEIVE to a state of RECEIVE only. Placing a call on HOLD is an example of a basic supplementary feature changed from having no standard method for implementation in the original SIP RFC to one that does have a standard method for innovation in the latest RFC-3261. However, many supplementary services do not have a standard SIP implemen- tation, and a great deal of interworking problems occur as customers transition from TDM trunks to SIP trunks with the expectation of these supplementary services working as efficiently and smoothly as they did in TDM networks. Summary IP Telephony can be viewed in two distinct ways. One is as an exact replacement for tra- ditional telephony with the change of using packet-based networks as a replacement for the traditional TDM networks. This initial but limited view of IP Telephony is the first way that most users look at this transition. The second and more dramatic way to look at IP Telephony is as a wholesale change in the method of using communications technologies. The phone transitions from a device that is only about speech to a device that can be used to initiate and manage many different methods of communications. Finally, the ultimate transition of IP Telephony might be to a world in which little distinc- tion exists between a standard phone and any other general purpose communicating device. The future of communications will allow for endless possibili