What is referred to as Centrex when a CO switch provides business telephone switching services?

Centrex refers to a service provided by a Central Office (CO) switch, where the telephone switching functionalities are handled by the telephone company rather than being managed on-site by a business. This system allows businesses to utilize advanced telephony features such as call forwarding, voicemail, and conference calling without the need for complex and costly on-premise equipment.

This concept enables organizations to modify their telecommunications setup as their needs change, all managed through the provider. Businesses benefit from the scalability and reduced maintenance responsibilities associated with using this service. The term "Centrex" specifically highlights its relation to central office provisioning of services, which is key to understanding how this system operates in the context of business communication needs.

On the other hand, the other options represent different telecommunications concepts. Trunks are connections that carry multiple calls between switching systems. A key system refers to a more traditional and simpler method of managing internal calls and is typically hardware-based. An FX line typically designates a Foreign Exchange line, which is used for connecting a local phone system to a distant CO. Hence, Centrex stands out as the correct term that encompasses the concept described in the question.