One-way audio on VICIdial calls and how to find the cause

The agent can hear the lead but the lead hears nothing, or the reverse. Calls connect, timing is fine, and only one direction of audio is missing. That is a classic asymmetry: media is flowing one way and being dropped the other. Almost always it is RTP, the media stream that carries voice, not getting through in a single direction.

Because RTP travels separately from SIP (Session Initiation Protocol) signalling, one direction can succeed while the other is blocked. The fix starts with figuring out which leg is silent.

Where the asymmetry comes from

Each end of a call announces an address where it wants to receive audio. If the box sits behind any address translation and advertises a private IP, the far end sends packets to an unroutable destination, so that one direction dies. A firewall that allows outbound RTP but not inbound, or the reverse, produces the same pattern. Symmetric RTP, sending audio back to the address packets actually arrive from rather than the address advertised, repairs many of these cases.

The reason this is so common on cloud boxes is that the carrier and the agent often live on opposite sides of the network boundary. The leg inside your network frequently works fine because nothing is translated, while the leg crossing the boundary breaks. That is why one-way audio so often shows up only on real outside calls and never in a local test between two extensions.

sequenceDiagram
  participant Agent
  participant Box
  participant Carrier
  Agent->>Box: RTP up
  Box->>Carrier: RTP up
  Carrier-->>Box: RTP down blocked
  Box-->>Agent: no return audio
  Note over Box,Carrier: agent hears nothing

Isolate the silent leg

1. Decide who can't hear. If the agent hears the lead but the lead hears nothing, the path from the box out to the carrier is blocked. If the lead hears the agent but the agent hears nothing, the return path into the box is blocked. Pin this down first; it tells you which firewall direction to inspect.
2. Check the external IP and NAT settings. If the box advertises a private address, return audio has nowhere to go. Set the external address and local network correctly. The NAT traversal config is the single most common cause of one-way audio on a box behind translation.
3. Open the RTP range both ways. A half-open firewall, inbound allowed but outbound denied or the reverse, is the textbook one-way symptom. Confirm the UDP RTP range passes in both directions on every device in the path.
4. Enable symmetric RTP. Telling Asterisk to send media to the source of incoming packets, not the advertised IP, fixes a NATed SIP trunk that lies about its address.

Once you know which leg is silent, the firewall or NAT setting to change is obvious. For the full diagnostic order, see the troubleshooting playbook. If the audio is missing in both directions instead of one, start with no audio on VICIdial calls. To skip NAT tuning entirely, VICIfast ships a box with the public IP and RTP path already configured.