Dial-in is a service that assigns multiple phone numbers to a single telephone line, routing incoming calls to different extensions or departments based on the dialed number. Provided as an NTT landline service, it is widely used in corporate offices in combination with PBX systems.
The mechanism works by the exchange notifying the PBX of the incoming number (dial-in number), and the PBX ringing the corresponding extension phone. For example, if 03-XXXX-1001 is assigned to the sales department and 03-XXXX-1002 to accounting, external callers can reach the desired department directly. This eliminates the need to go through a main number and ask to be transferred, reducing caller wait times and receptionist workload.
Cost-wise, the major benefit is adding numbers without installing additional physical lines. Normally one line supports one number, but with dial-in, dozens of numbers can operate on a single line. However, simultaneous call capacity is limited by the number of physical lines - even with 10 numbers assigned, only 2 simultaneous calls are possible with 2 lines. Accurate line capacity estimation is important for departments with concentrated incoming calls. The fee structure is approximately several hundred yen per month per dial-in number, added to the line's base fee.
Dial-in is often confused with extension numbers. Extension numbers are managed by the internal PBX and cannot be dialed directly from outside. Dial-in numbers are "external numbers" recognized by NTT's exchange, fundamentally different in that they can receive direct incoming calls from outside. With the spread of cloud PBX, dial-in equivalent functionality can now be achieved via the internet without depending on physical lines, making cross-location number integration and remote work support easier. Review business phone environment setup in the business phone number guide.