The system can be configured to allow you to answer incoming calls directed to other users. When you hear an incoming call alerting for a different user (or you detect such a call via other means), you can dial a special number to answer that call.
By default, the Router records direct all calls made to 8nnn to the pickup application. That application intercepts the incoming call pending for the user whose Account alias is nnn. I.e. to pick up an incoming call for the user Account j.smith whose numeric alias is 245, you need to dial 8245.