The "0" Was a Signal for Long-Distance Calls
Japanese phone numbers starting with 0 is no accident. The leading 0 is called the "trunk prefix" and tells the exchange that an area code follows. In the landline era, local calls only needed the local exchange number plus subscriber number. The 0 was added only for calls outside the local area, functioning as a "long-distance signal."
This system was established during the 1960s nationwide automatic dialing plan. Before that, long-distance calls required asking an operator to connect you manually. With automatic exchanges, users could dial directly, but the exchange needed a way to distinguish local from long-distance calls. The digit 0 was chosen for this purpose. This design has been maintained for over half a century. Books on telephone history cover the evolution of telecommunications technology in detail.
Why "0" Was Chosen
Dial Pulses and Digits
On rotary phones, dialing a number sent pulse signals to the exchange corresponding to the rotation distance. The digit 1 sent 1 pulse, 2 sent 2 pulses, and 0 sent 10 pulses. Since 0 had the most pulses, the exchange could reliably identify it as "not a normal local number prefix." Local exchange numbers started with 1-9, so a leading 0 immediately indicated a long-distance call. This simple but robust design minimized misrouting risk.
International Convention
Using 0 as the trunk prefix is not unique to Japan. Many countries including the UK, France, Germany, Australia, and South Korea also use 0. The US and Canada use 1 as their long-distance prefix under the North American Numbering Plan (NANP), with 0 reserved for operator assistance.
Why Area Code Lengths Vary by Region
In Japan's numbering system, area codes range from 2 to 5 digits. Tokyo (03) has 2 digits, Yokohama (045) has 3, Musashino (0422) has 4, and Hachijojima (04992) has 5. This reflects the number of telephone subscribers in each region.
Landline numbers are fixed at 10 digits total. Shorter area codes leave more digits for local exchange and subscriber numbers, accommodating more phone numbers. Tokyo's 2-digit code (03) theoretically supports 100 million numbers. Remote islands with few subscribers use 5-digit codes to conserve number space. See History of Phone Number Digit Lengths for the full evolution.
The "0" in Mobile Numbers Is Different
The leading 0 in mobile numbers (090, 080, 070) serves a different purpose than the landline trunk prefix. Mobile phones have no concept of "local" vs. "long-distance," so the 0 simply identifies the number band. The 0 in 090 tells the exchange "this is a mobile phone number."
The rule of dropping the leading 0 for international calls applies to both landlines and mobiles. +81-3-1234-5678 (Tokyo) and +81-90-1234-5678 (mobile) both drop the 0 because it is a domestic-only prefix unnecessary for international routing. See Guide to Japan's Country Code +81.
Is the "0" Still Needed in the IP Era?
With NTT's landline network fully migrated to IP in January 2025, physical exchanges have disappeared. On IP networks, routing is handled by software, so technically the leading 0 is unnecessary. Yet the 0 prefix persists because the cost of changing a numbering system embedded in phone books, business cards, websites, databases, and government documents across all of society would be enormous. The leading 0 has transitioned from a technical necessity to a social convention, and will likely remain in use for a long time.
What the Leading Digits Tell You
The digit after 0 reveals the call type: 01-09 are regional landlines, 050 is IP phone, 070/080/090 are mobile, 0120/0800 are toll-free (Free Dial), 0570 is Navi Dial, and 0990 is premium-rate information. This system is strictly managed under the MIC's Telecommunications Numbering Plan. When you receive a call from an unknown number, checking the first few digits gives you a quick read on the caller's nature.
Fun Facts About "0"
On rotary phones, dialing 0 sent 10 pulses - it was treated as "10th" rather than "0th." Digits 1-9 corresponded to 1-9 pulses, and 0 was assigned 10 pulses. In the rotary phone world, 0 was effectively "10." This design arose because early electromechanical exchanges counted pulses to identify digits, and 0 pulses would be indistinguishable from "no signal." Assigning 10 pulses to 0 was an ingenious workaround. Today's numbering system is built on top of these historical technical constraints.