No audio during VoIP calls

Sometimes it happens that there is no audio during a phone call. The call itself is established, but there's no sound. It's also possible that the audio only works in one direction.

The cause can almost always be traced back to poor NAT handling by the router. It could also be that there are multiple NAT routers placed in sequence. Another possibility is that the firewall settings are too strict.

Possible solutions

• Ensure that phones are connected directly to the modem/router; avoid multiple routers.
• Restart the modem/router. This often helps.
• Lower the security level of the firewall.
• If the problems persist, consider placing the modem in bridge mode and using a MikroTik router.
• Check if SIP_ALG is disabled on the modem.
• Check if the firewall is configured correctly.

Audio drops after 15 minutes

• It's possible that the router/firewall has terminated the SIP connection (not the audio/RTP) because there has been no traffic for a while. After 15 minutes, we check if the SIP connection is still functioning, and if we don't receive a response, we terminate the audio. At that point, technically, there is no call anymore. You cannot put that call on hold or transfer it because an intermediate device has disconnected it. Therefore, check if a form of Keep alive is enabled on your device. You can find the exact settings on the wiki page for your device.
• It's also possible that the Session Timer is enabled on the device. If so, disable it. It's not necessary to have this enabled on our platform.