Caller ID allows you to see the number and/or name of the caller before you answer the phone.

The caller ID feature is useful if you only wish to take calls from people you know, as many devices allow you to enter the callers number and associate a name to this which is shown when a call is received. This allows easy recognition of the person calling.

The Caller ID service is enabled by default, all you need is a softphone/hardphone that is capable of showing the Caller ID. The majority of VoIP phones have this functionality, including many VoIP telephone adapters.