One way audio problems
One-way audio is a common VOIP problem, and there are many possible causes.
Firmware
Outdated firmware in routers, VOIP phones, firewalls, etc. can cause one-way audio. Ensure you have the latest updates for all the devices in the call path.
Configuration
Particularly if NAT is involved in the call path, configuration of the various devices may be a problem. Check to see if all devices are configured appropriately for your environment.
Finding the Cause The basic troubleshooting technique is to use a tool like Ethereal to capture SIP and RTP packets at each point in the call path where packets could be lost. Interpreting the resulting captured packets requires some familarity with how networking and VOIP work.
For example, if the call path is:
VOIP phone/device --< a >-- firewall --< b >-- sip proxy --< c >-- firewall --< d >-- asterisk
Troubleshooting Steps
Start capturing at point A. Make a VOIP call that will have one-way audio. Analyze the capture. If a problem found, fix it and retest. Otherwise move the capture point to the next point (a, b, c, d, etc) and start over.
If the problem is intermittent, then a long-term simultanous capture at multiple points can be used to attempt to capture a complete call with the problem. Most capture tools will let you capture only traffic from selected devices, so the volume of captured information can be kept to a reasonable size. If a back-to-back SIP user agent (for example a Session Border Controller) is part of of the call path, then it may be necessary to capture all VOIP traffic at some points to ensure catching the wanted call since the IP addresses can change when traversing this device.
