I am using a Snom phone. The CyberData device does not stop ringing if it misses a point-to-point SIP call.
By default, the Snom phone initiates a point-to-point SIP call on a port that CyberData devices do not monitor. This causes missed communication between the phone and the device, and the device will not recognize that the phone has hung up.
To resolve this issue, configure the phone to use the standard port for SIP calls, port 5060.
Follow these configuration instructions, using the attached screenshots as a reference:
Log in to your Snom phone's web configuration page.
Under “Setup”, click “Advanced”.
Click on the “SIP/RTP” tab.
Find the setting “Network identity (port)” and enter 5060.
Scroll to the bottom of the page and click “Apply”
When the page reloads, click the “Reboot” button that appears at the top of the page.
This was tested with firmware snom320-SIP 184.108.40.206.