Firmware 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.
Please note that the default value of this setting is changed: before 188.8.131.52 this setting is off by default, after 184.108.40.206 the default value is on.
Firmware Version 10.x
Starting with version 10.1.33.33, if the following settings are configured:
The number of the second caller appears on the screen.
Example on color display
The number of the second caller (in the example below 34081) appears in the lower side of the screen like this: (see the red arrow)
Example on D120
The number of the second caller (in the example below 3401) appears in the upper side of the screen like this: (see the red arrow)