Which of the following is a common root cause of one-way audio in VoIP?

• A. NAT/firewall issues, asymmetric routing, or media address/codec mismatch
• B. Excessive CPU load on the endpoint
• C. Expiring certificates
• D. DNS SRV lookup failures

Answer: (A)

One-way audio in VoIP happens when the media path can travel in only one direction while the return path is blocked or misrouted. When a device is behind NAT or a firewall, the IP addresses and ports used for the media stream in the session description are often not reachable by the other side. The remote party sends RTP to an address that the network cannot reach, so the audio only travels in one direction and the other side never hears it. Asymmetric routing compounds this, because the outbound media may exit through one network path while the return path follows a different route that is blocked by a firewall or NAT, preventing the reverse audio stream from arriving. A mismatch in the advertised media address or the chosen codec can also cause the receiving end to fail decoding or even reject the incoming stream, resulting in no audio in one direction. Other issues like expiring certificates or DNS service discovery problems disrupt call setup or service location rather than the ongoing media path once a call is established, so they’re not the typical root cause of sustained one-way audio.