The call can be connected but there is no audio at all! This is a tricky issue frequently encountered in projects, and the specific troubleshooting steps are as follows:
First, check the basic network connectivity and audio devices. Confirm that the network cable of the IP phone is securely plugged in, then inspect the hardware: check if the handset cord is loose and if the headset or speaker port is fully connected. In many cases, no audio is caused by such minor physical connection issues, so eliminate this possibility first.
Second, this step is particularly critical: it is essential to determine whether the audio issue is one-way or two-way silence. If one party can hear while the other cannot, the problem is usually with the audio sending path of the device on the silent side or the corresponding network transmission link. If neither party can hear the other, focus on checking the network path between them, especially devices such as firewalls and routers, which are likely blocking the voice media stream.
Third, verify the network firewall and NAT settings. Note that the voice stream (i.e., RTP media stream) for IP voice calls and the signaling use different channels. Ensure that the firewall has opened the port range required for the RTP media stream. In addition, check if the NAT traversal configurations (e.g., the SIP ALG function) are enabled or configured correctly—incorrect settings here are often the root cause of no audio issues.
Fourth and finally, confirm that the audio codecs are negotiated consistently. The IP phones on both ends of the call must support and use the same audio codec, such as the common G.711 and G.729. If codec negotiation fails between the two parties, the call can be connected (normal signaling connection) but with no audio (no media stream). This step requires verifying the codec parameters in the device configuration interface.
If the problem is still not identified after completing the above four steps, use packet capture tools for in-depth analysis to check whether the RTP stream is sent and received normally.