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Different types of voice communications can result in different voice quality issues, as follows:

• PC-to-PC Calls.During a PC-to-PC call, which can include either a Microsoft Office Communicator Universal Serial Bus (USB) phone or a stand-alone IP phone that is running Microsoft Office Communicator 2007 R2 Phone Edition, computers send audio packets to each other over the network by using the Real-time Transport Protocol (RTP). The digital nature of the audio stream prevents audio modification by the network, but audio packets can be lost or delayed. Delays and losses can degrade voice quality when the receiving computer decodes and reconstructs the audio signal. Microphones, speakers, and headphones that are attached to the computer can also cause audio distortion. Introduction of distortion on both the sending and receiving computer can make diagnosis more difficult.
  
  
• PC-to-Analog Phone Calls.During a PC-to-analog phone call, a computer sends audio packets to the Office Communications Server Mediation Server, which then sends the packets to an IP/PSTN gateway. The IP/PSTN gateway and the PSTN network to which the gateway routes the packets convert the digital audio signal to an analog signal that is received by a PSTN phone.
  
  The IP network transmission from the computer to the Mediation Server is similar to the network transmission between computers in the PC-to-PC call scenario, so the potential issues are similar. The voice transmission from the IP/PSTN gateway to the PSTN phone introduces different issue areas that can affect voice quality, including incorrectly configured IP/PSTN gateway settings, potential electronic interference with the analog signal, and poor performance of PSTN devices (for example, mobile phones and PSTN audio conference bridges). The potential for both digital and analog issues can make diagnosis of PC-to-phone issues more difficult than the diagnosis of PC-to-PC issues.
  
  

  Note:
  PC-to-phone calls also include Office Communicator 2007 R2 calls to a PSTN audio bridge. The PSTN audio bridge appears to the Office Communicator client as a PSTN endpoint.

• Audio Conferencing.In an audio conference, multiple clients are connected through the Office Communications Server 2007 R2 A/V Conferencing Server. Because multiple clients can participate in an audio conference, isolating an issue can be a more complex undertaking. Users can participate in the audio conference from either a PC or an analog phone, so the issues that are associated with both PC-to-PC and PC-to-phone connections are also potential issues for audio conferencing. The performance of external PSTN audio conference bridges in creating echo or latency in audio can also be a factor and is typically outside the control of the Office Communications Server administrator.