Incoming calls to a company first pass through the CO. The CO sends calls to the communication server of the company. Trunks are phone lines that carry calls between two servers, between a CO and a server, or between a CO and a phone.

The CO receives the dialed digits from the caller, processes the digits, and seizes a trunk that is assigned those digits. From the seized trunk, the CO sends a continuing transmission to the destination phone or communication server. No other calls can be sent over that trunk until the current call disconnects.

As a trunk can carry only one call at a time, trunk groups are usually created and are assigned to the same digits. With a trunk group, the CO receives the digits of a dialed phone number and checks if any trunk assigned to that number is available. The CO then seizes an available trunk. You can make as many calls simultaneously over a trunk group with the available trunks. A trunk group, therefore, can carry multiple calls for the same phone number. When a trunk group carries incoming calls, that is, calls made outside the company communication server location to the communication server, the communication server connects the calls to their proper destinations within the company.

In addition to connecting incoming calls to the proper destinations, communication servers act like private COs for company employees. Employee phones connect to a communication server by phone lines called extensions. You can assign numbers to extensions and the numbers become the employee phone numbers for internal calls.