Version 8.x

When receiving the incoming call you can press the button on the right corner to see the call waiting number:

If you want to show the caller-id of the waiting call on the phone display you have to disable the setting always_show_active_call. This setting isn't manageable via Web interface, if you want to change it you should use provisioning or single setting HTTP request.

Please note that the default value of this setting is changed: before this setting is off by default, after the default value is on.

Version 10.x

Starting with version, if the following settings are configured:

call_waiting = on or visual only 

The number of the second caller (34081) appears in the lower side of the screen like this: (see the red arrow)