The short answer: Japanese phone numbers start with 0 because the leading zero is a trunk prefix, a signal to the telephone exchange that an area code follows and the call is leaving the local area. That is also why you drop the 0 when dialing a Japanese number from abroad and use the country code +81 instead.
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 it tells the exchange that an area code is about to follow. In the landline era, a call within the same local area needed only the local exchange number and the subscriber number. The 0 was required only when the call left the local area, which is how it came to work as the signal for long-distance dialing.
This arrangement took shape as automatic switching, which connected long-distance calls without an operator, spread across the country. Before that, placing a long-distance call meant asking an operator to put it through by name and number. Automatic exchanges let subscribers dial anywhere in Japan themselves, but the equipment still needed a way to tell a local call from a long-distance one, and 0 became that marker. As of 2026 the design remains in place: a landline call that leaves the local area begins with 0.
Why "0" Was Chosen
Dial Pulses and Digits
On a rotary telephone, turning the dial sent a train of pulses to the exchange in proportion to how far the dial traveled. The digit 1 sent one pulse, 2 sent two, and 0 sent ten. Because 0 carried the largest number of pulses, the exchange could recognize it with confidence as something other than the first digit of an ordinary local number.
Local exchange numbers began with 1 through 9, so a leading 0 marked a long-distance call immediately. The scheme was simple yet remarkably robust, and it kept misrouting to a minimum. When exchange hardware had very little processing capacity, judging the type of call from a single leading digit was a sound engineering choice. Those early exchanges were built from relay circuits that counted pulses and threw switches mechanically, so the decision logic had to stay as simple as possible.
International Convention
Using 0 as the trunk prefix is not a Japanese invention. ITU-T Recommendation E.164 leaves the domestic trunk prefix to each country, and a great many of them settled on 0. The United Kingdom, France, Germany, Australia, and South Korea are among the countries that dial 0 for calls outside the local area.
The United States and Canada, by contrast, use 1 as the long-distance prefix. That is a feature of the North American Numbering Plan (NANP), which reserves 0 for operator assistance. China uses 0 for domestic long-distance calls and 00 for international calls, a structure close to Japan's.
Why Area Code Lengths Vary by Region
In Japan's numbering system, the length of an area code depends on the region. Counting the leading 0, Tokyo (03) has two digits, Yokohama (045) three, Musashino (0422) four, and Hachijojima (04992) five. The differences follow from how many telephone subscribers each area has to accommodate.
A landline number is fixed at ten digits in total. The shorter the area code, the more digits are left for the local exchange number and the subscriber number, and the more numbers the area can hold. The 23 wards of Tokyo combine a two-digit area code (03) with a four-digit exchange number and a four-digit subscriber number, which allows 100 million combinations in theory. Remote islands with few subscribers take the opposite route and use a five-digit area code to conserve number space.
This closed numbering plan, in which the total number of digits is fixed, lets an exchange decide that a number is complete simply by counting digits. History of Phone Number Digit Lengths traces how those digit counts changed over time.
The "0" in Mobile Numbers Is Different
The leading 0 in a mobile number (090, 080, and 070) does a different job from the landline trunk prefix. Mobile service has no notion of local versus long-distance, so here the 0 simply identifies the number band. The 0 in 090 tells the network that the number belongs to a mobile phone.
Dropping the leading 0 for international calls matters just as much for mobiles as for landlines. In +81-3-1234-5678 (Tokyo) or +81-90-1234-5678 (mobile), the 0 disappears once the country code +81 takes over, because the 0 is a domestic prefix that international routing does not need. See Guide to Japan's Country Code +81 for more.
Is the "0" Still Needed in the IP Era?
NTT completed the migration of its landline network to IP in January 2025, and the older switches that used to relay long-distance calls have finished their work. On an IP network the routing is handled in software, so technically a call could be connected without the leading 0. The prefix survives because replacing a numbering plan that has been in place for decades would cost an enormous amount.
Phone numbers are printed in directories, on business cards, on websites, in databases, and in government records, and changing all of them at once is not realistic. A change to the numbering plan would ripple through the country's infrastructure as a whole, which puts it beyond a purely technical decision. The leading 0 remains, its role having shifted from technical necessity to social custom.
What the Leading Digits Tell You
The digits that follow the 0 give a rough read on the kind of line you are dealing with. 050 is an IP phone number, 070, 080, and 090 are mobile, 0120 and 0800 are toll-free (Free Dial), 0570 is Navi Dial, and 0990 is a premium-rate information service; a number that starts with 0 and fits none of these is a regional landline. The uses of each number band are set out in the Telecommunications Numbering Plan issued by the Ministry of Internal Affairs and Communications, and a carrier has to be assigned numbers by the Minister before it can use them. When an unfamiliar number calls, the first few digits alone tell you a good deal about the caller and help you decide whether to call back.
Fun Facts About "0"
There is one more curiosity hidden in the 0. Turning the dial to 0 sent ten pulses because 0 was treated as the tenth position rather than the zeroth. Early electromechanical exchanges identified digits by counting pulses, and a count of zero pulses could not be told apart from no signal at all, so 0 was given ten pulses instead. In the world of the rotary telephone, 0 was effectively a 10. Constraints like these, stacked one on another, became the foundation of the Japanese numbering plan.