Many clamshell phones have secondary external displays on the outside, so that the user can see who is calling before answering in incoming call.
The display therefore shows Caller-ID information during an incoming call. Most phones display the time and/or status at other times.
Newer phones have 2nd displays that are color, which allows associated photos to be shown for incoming callers, a feature known as Picture ID or Photo Caller ID.
In many camera phones with full-color 2nd displays, the 2nd display can be used as a viewfinder, to frame self-portraits.