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 188.8.131.52 this setting is off by default, after 184.108.40.206 the default value is on.
Starting with version 10.1.33.33, if the following settings are configured:
The number of the second caller (34081) appears in the lower side of the screen like this: (see the red arrow)