Some telephones intended for use as extensions have built-in intercom features; a key telephone system for a small business may offer two to five lines, lamps indicating lines already in use, the ability to place calls on 'hold' and an intercom on each of the multiple extensions.
For inbound calls, a switchboard operator or automated attendant may request the number of the desired extension or the call may be completed with direct inbound dialing, if outside numbers are assigned to individual extensions.
Since not all users dial out at the same time, sharing trunk lines is cheaper than giving each phone its own private landline.
Extensions make it easier for workers to bring their telephone number with them if they change offices in the building.
Extensions allow personalized traffic and call accounting data to be captured by the PBX or a phone recorder for cost control.