Your phone may or may not be part of one or more call pick-up groups. If it is, then this feature will allow you to pick up a call that is ringing at another phone that is a member of the same call pick-up group(s). This allows someone to answer the phone of someone else who is not available for a call.
To do this, while the other phone is ringing, press *8 and then pick up the handset, or press the DIAL, speakerphone, or headset button.