One-way Audio

If you are experiencing a problem wherein you can hear a caller, but they can't hear you (or vice-versa) you are very likely experiencing a NAT issue.

The process to resolving a NAT issue is:

Power cycle networking equipment and phones
Check and correct phone configuration
Check and correct router and network equipment configuration
Check and update router and network equipment firmware
Replace network equipment with a recommended device

Phone configuration:

NAT (network address translation) support must be enabled on your handset in most situations. On the Cisco and Linksys phones this can be found on the "Ext X" tab's. "NAT mapping enable" and "NAT keepalive enable" should both be checked.

Router Configuration:

A common cause of NAT issues is network and security devices - such as routers and firewalls.

SIP ALG configuration should be checked on the device. In most cases it should be OFF, but in certain configurations on certain devices it should be turned ON

Test with SIP ALG disabled
If you experience issues, try testing again with SIP ALG enabled

Check that your router or firewall is running the latest firmware version
Rebooting your router or firewall appliance can often temporarily fix one-way audio problems.
If you continue to experience problems with your firewall or router, you should replace it with an appropriate supported device
Ensure that your firewall or router is powerful enough to support the number of handsets and users on the network