Trunks are line-side circuits that connect a central office to a customer's premises-based switching system (PBX). Trunks are available as analog or digital, and may be configured as:

• Inbound, which provides for termination of incoming calls at the attendant console
• Outbound, which provides one-way outgoing service for direct dialed calls
• Combination, which provides both incoming and outgoing call capability

Flat rate trunks offer cost savings due to the "pooled" nature of the facilities. Measured rate (usage-based) trunks are designed for PBX customers with heavy inbound traffic and light outbound traffic.