1. Real-time Transport Protocol (RTP). Customers normally request for audio on their internal IP address for example 192.168.1.1. Such requests will make it impossible for the underlying carriers to send audio to customers. Audio must always be requested using a public IP address and if you are experiencing one-way audio, the reason could be that you are requesting an internal IP.
2. Time Mismatches. When making calls, each side of the call must be able to send an RTP within the same codec and each of the sides must also have the same timing. In simple terms, this means the time interval for RTP to transmit packets. Phase timing is also known as ptime and the most common ptime is 20 among many customers. If you have a situation where the ptime transmission is 10 and the other ptime is 20, you will encounter garbled speech on either side.
3. Codec Mismatch. To have two-way audio on each side of the VoIP call, all calls must have the same RTP within the same codec. Let us an example of a call sending a codec of G.711u on one side and the other is sending G.729. These two callers will not be able to hear one another. It is like two people trying to communicate but they are speaking different languages.
4. Comfort Noise. This is another issue that has been found to cause distorted noise or choppy audio. You can improve on your VoIP audio by just disabling this setting and all your calls will be clear again.
5. Issues on the public internet. When you have two IP addresses that are on a public internet exchanging audio directly, you will experience audio issues. This is because of the customer's carries and the underlying carries. There is always a possibility that the two networks will have issues, especially on the public internet.
6. Blank RTP Packets. If you have been frequently sending blank RTP packets, you will at the end of it all experience audio issues because no audio has been sent in actual sense. So, if you are experiencing audio issues, it is always good to confirm if you are sending blank RTP packets.
7. RTP blocked by a firewall. You should always ensure that your firewall does not block your audio network because this will result in one-way audio. This is a very common problem among many customers.
8. SIP ALG setting. If you are experiencing audio issues, you should consider disabling the SIP ALG settings. This setting normally changes the IP address of your audio. This setting can rewrite an internal IP address that is intended for audio.
9. Closed ports/wrong ports. You should always confirm if the correct ports are open on your network. This is because you could be preventing your audio from flowing properly which leads to one-way audio. The same thing can occur if you are sending audio to the wrong port, something that you did not agree upon during SIP communication.
10. Latency. There are high possibilities that other traffic on a local network could affect your VoIP traffic. Make sure to check if there are such issues because it is very common. The most common causes of VoIP audio issues are jitters and latency.