An extension line is an internal phone line within an organization connected through a PBX. Dialing a short extension number (typically 3-5 digits) connects to colleagues without using external lines, making intra-office calls free. "Extension 2345, please" is a daily occurrence in Japanese offices.
Key features include free inter-extension calls, transferring external calls to other extensions, "park hold" for picking up held calls from different phones, "pickup" for answering absent colleagues' calls, and multi-extension conference calling.
In the 2020s, the spread of cloud PBX reshaped what an extension means. Extensions had previously been confined to a physical office, but they became usable from home or on the road through smartphone apps. Remote staff can place and receive calls under the company's main number, and extension transfers work just as they do at a desk phone.
Extension numbering design is fundamental to corporate telephony. Typically, number ranges are assigned by department (e.g., Sales 1xxx, Admin 2xxx), with main number calls routed through IVR department selection to appropriate extension groups. See workplace call handling and business phone guide for more.