One-way audio is commonly caused by which of the following?

• A. NAT/firewall issues, asymmetric routing, or media address/codec mismatch
• B. DNS resolution problems
• C. Server overload
• D. Voice activity detection misconfiguration

Answer: (A)

One-way audio in VoIP usually comes from problems with the RTP media path. When signaling negotiates where to send media, the actual media packets must traverse to the other party and return. If NAT or a firewall alters addresses or blocks the RTP ports, the first party’s audio can reach the other side while the return audio can’t, so you hear audio in only one direction. Similarly, asymmetric routing—where the path for incoming and outgoing media goes through different devices or networks—can cause the wrong path to be used or blocked by stateful devices, leaving one side silent. A mismatch in the media address or codec negotiated in the session description can also prevent the receiving side from properly decoding or routing the audio, leading to no audio coming back.

DNS resolution problems usually affect call setup or routing rather than the media path itself, so they don’t typically cause one-way audio. Server overload tends to degrade overall quality, cause delays, or drop calls rather than isolate audio to only one direction. Voice activity detection misconfiguration affects how silence and voice are detected, not the fundamental path of the media stream.

In practice, diagnose by checking NAT/firewall traversal for RTP, ensuring both sides see the correct media IPs/ports in SDP, and verifying codec compatibility and path symmetry.