If no audio is heard when calling a remote SIP device, what could be a potential issue?

When no audio is heard during a call to a remote SIP device, a common issue is that the RTP (Real-Time Protocol) traffic is being blocked by a firewall. RTP is responsible for delivering audio streams over the network. If firewalls or network security settings are improperly configured, they can impede RTP packets, resulting in one-way or no audio during a conversation.

In SIP communications, two distinct types of packets are used: SIP signaling packets for initiating and managing calls, and RTP packets for the actual media (audio or video). While SIP packets might successfully traverse the network and establish a connection, if the RTP packets are blocked, the media stream cannot be established, leading to silence even though the connection appears active.

While issues like improper registration of the SIP device, an unanswered call, or malfunctioning software can lead to problems in the call process, they do not specifically account for the absence of audio after a call has been established. The focus on RTP blockage directly links to the symptoms observed—silence during an active call despite the signaling process being intact.