Call Forking, also known as Call Splitting. A feature of the Session Initiation Protocol (SIP) that enables an incoming call to ring several extensions, i.e., other phones, at once. The first phone to answer takes control of the incoming call.
Call Forking enables an extension number to be registered by multiple SIP endpoints simultaneously. A single call can ring many endpoints at the same time (Ring Group, ONA, Virtual Office Mobile, Virtual Office Desktop, etc). With SIP forking, you can register one extension number to multiple SIP endpoints, so to let your desk phone ring and mobile softphone ring at the same time, and you will be able to take calls from either device.