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.
Cloud PBX has transformed extensions. Previously limited to physical offices, extensions now work from home or on the go via smartphone apps. Remote workers can make and receive calls using the company's main number, with extension transfers functioning normally.
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.