hunting

In telephone switching systems, hunting is the automatic process of testing successive telephone lines in a group until an idle one is found, then routing an incoming call to it. When a caller dials a number served by multiple lines, the exchange does not simply pick a random line or overload the first one: instead, it systematically searches through the available circuits in a defined sequence until it finds one not in use, then connects the call there. This prevents calls from being rejected when some lines in the group are busy while others are free.

Hunting works through electromechanical stepping mechanisms in older exchanges or through software logic in digital systems. The simplest form, called linear hunting, tests lines in strict numerical order: line 1, then 2, then 3, and so on, wrapping around to the start. Circular hunting begins from the last line that received a call, reducing wear on the earliest lines in the group. Some systems use random hunting to distribute calls more evenly, or load sharing to route calls based on current queue depth or call duration estimates.

Variants and application

Hunting is essential wherever multiple lines serve a single directory number: business switchboards, emergency services, airline reservations, and customer service centers all rely on it. The size of the hunted group can range from two lines up to hundreds in large call centers. The hunting algorithm and group membership are typically configured during exchange installation and modified as demand changes. Poor hunting configuration can result in systematic overloading of certain lines while others remain idle, creating bottlenecks that callers experience as repeated busy signals.

The term comes from the search or hunt for an available resource, analogous to hunting game. It dates to the mechanical step-by-step and crossbar switch era, where physical relays and wipers actually stepped through terminals until finding an open path. In modern digital exchanges and VoIP systems, the same logical function persists, even though no mechanical hunting occurs: the principle remains that an idle circuit must be located and claimed before a connection can be made.

Hunting differs from queuing: a queued call waits in a buffer until a line becomes free, while a hunted call is either connected immediately or rejected with a busy tone if no free line exists at that moment. Some modern systems combine both, queuing calls when all lines are busy rather than immediately refusing them. The hunting group, the algorithm used, and its interaction with call waiting, call transfer, and conference bridging are crucial parameters in exchange and PBX design.