One Way Audio and the Inability to Receive Incoming VoIP Calls

As a VoIP Service Provider, you will no doubt have heard customers complaining about these two problems. In one case, the customer makes a call or Receives an incoming call and can hear their calling party. However, the calling party complains they cannot hear you. This is possibly because the VoIP Endpoint phone has a problem or there could be routing problem in the network.



More commonly, this is a problem with the firewall belonging to the enterprise. The enterprise network Administrator prefers not to allow incoming data sessions and for the VoIP session, the incoming RTP uses a different port to the outgoing RTP. The UDP port number used by RTP is negotiated by the SDP inside the SIP signaling at the commencement of the session or phone call. The actual details are subject to the implementation and configuration of the network and sometimes outgoing and incoming UDP ports for the RTP may be dissimilar.



Re-configuring the firewall and ensuring that the RTP port is the same for incoming and outgoing RTP streams will rectify this problem.

A similar problem, but more difficult to isolate, is where a customer complains they can make outgoing calls but their colleagues say they called your customer but their VoIP phone did not ring. The user can make outgoing calls. Failure of the incoming call is caused by REGISTRATION of their VoIP phone having expired on its registration server.


When a VoIP endpoint successfully registers on the registration server or the network, the registration server includes in its response, a time frame during which the REGISTRATION is valid. The VoIP endpoint/phone should RE-REGISTER with the registration server before half of the time period has expired.

Frequently, there is an imbalance between when the endpoint believes it should renew the registration and when the registration server expires the registration. A tool which can track the state of registration [New, Unauthorized, Expired or Gone] is invaluable in being able to find these stealth problems.



Wireshark will show you the time and relationship between such REGISTRATION messages. However, an advanced intelligent Service Assurance tool such as Palladion will track the state of each endpoint, and also the firmware release of each endpoint, allowing you to find these anomalies in seconds.