one-way audio issues in VoIP is best done one step at a time. Start
by eliminating any double NAT possibilities by disabling NAT on any secondary
routers that may be present on the LAN. This should be good basic LAN network design anyway and multiple instances of
NAT will cause problems both in VoIP and elsewhere.
One-way audio can occur in either direction, however, in-bound audio failure (Lack of audio from the outside caller reaching the inside network (LAN) phone.) is likely the most common. Many of these cases routers and firewalls cause audio not passing. Typically, the voice traffic (RTP) is not being passed to the correct SIP port for its journey inside the premise network.
Certain routers have been known to produce SIP issues, although many of
them have corrected issues with later models and later firmware. SIP ALG
or SIP transformations are notorious for causing issues with VoIP and can
be disabled on most routers for better VoIP quality.
You'll find below a link to a guide to where to find these settings in some routers. SIP transformations are known to corrupt some of the SIP headers resulting in issues with transferring voice traffic correctly.
*Note: These options could place the VOIP ATA in a venerable place for unwanted remote access. Use caution if ever leaving your VoIP PBX exposed to easy outside access.
If, after trying the above steps, you get good two-way audio voice, then the next step would be to
work inwards into your LAN to determining where the voice transmission is being
blocked. Once found, you
should be able to configure the device to allow needed VoIP packets to pass without breaking or changing the correct SIP
Some routers/firewalls use SIP transformations. This setting has been known to play havoc with some SIP headers and should be tried both off and on to see which way would be best for your VoIP. Typically we recommend that it be turned off, as with SonicWall devices. Past experience has shown them to cause problems. Other settings which should be looked at are in devices that do packet inspection. Often, they are the cause for some instances of one-way audio.
Turning Off SIP ALG (also called SIP Transformations) on some routers.