Almost all of us will meet this problem when we deploy our first VoIP network. We are often confused: why I cannot hear peer guy but he can hear me? why we cannot hear each other?

The root reason is that there is private network and public network. If both sides are in different network, the problem will happen. Please look at below figure which desribe a very simple VoIP network:

In this simple network, we have two VoIP devices, one is SIP phone whose number is 100, another is SIP client whose number is 101.

SIP phone is in a private network and its private address is 192.168.1.100, and its router is connected to public network with address 8.8.8.8.

SIP client is installed in one PC which is in the public network with address 8.8.4.4.

So when SIP phone makes a call to the SIP client, what will happen?

SIP phone say: Hi, I am 100, my audio address is 192.168.1.100. Please send audio stream to me.

SIP client answers it: ok. I am 101, my audio address is 8.8.4.4. Please send your audio to me.

SIP phone sends audio stream to SIP client. Since “8.8.4.4 ” is a public address, it is no problem for SIP client to receive the audio stream from SIP phone. That means SIP client can hear SIP phone now.

SIP client sends its audio stream to SIP phone “192.168.1.100”. You can see it is a private address and cannot be reached by SIP client which is in public address. SIP client will fail to send its audio stream to SIP phone in fact.

So finally, SIP client can hear SIP phone, but SIP phone cannot hear SIP client. This is a very typical one-way audio problem.

Then, how can we resolve it? To be continue …… 🙂