Voice & Communication

E.164

E.164 is an international standard that uniformly formats worldwide telephone numbers, enabling all countries' phones to interconnect using the +country-code-area-code-subscriber-number format.

E.164 standard international phone numbers ITU-T recommendation global phone numbers communication numbering system
Created: December 19, 2025 Updated: April 2, 2026

What is E.164?

E.164 is an international standard for uniformly writing telephone numbers worldwide. Set by the ITU-T (International Telecommunication Union), it enables all phones to interconnect. The format is “+country-code-area-code-subscriber-number,” for example “+81-90-1234-5678” for Japan. This standardization ensures VoIP, SMS, and international calls route correctly. It’s the foundation of secure and reliable communications infrastructure.

In a nutshell: “A worldwide rule where every phone number uses the same format so any country can use any number the same way.”

Key points:

  • What it does: Standardizes international phone number formatting
  • Why it’s needed: Enables accurate global communication routing
  • Who uses it: Telecom companies, communication system developers, multinational enterprises

Why it matters

E.164 matters for three reasons. First, it enables “interoperability”—uniform formats mean accurate calling from any country. Second, it ensures scalability—the 15-digit limit can handle world population growth without system overhauls. Third, security and compliance improve—standardized numbers ease fraud detection and compliance monitoring. With VoIP and IoT device proliferation, E.164’s importance grows.

How it works

E.164’s format is straightforward. The initial “+” indicates international dial prefix. The next 1–3 digits are the “country code” (Japan is 81). Following 1–3 digits represent “area code” indicating region or carrier. Finally comes the “subscriber number”—the individual phone number. The total is capped at 15 digits.

For example, calling from Tokyo to New York: +1-212-555-0123. The “1” is the US country code, “212” is New York’s area code, and the rest is the subscriber number.

Real-world use cases

International business Multinational companies manage employee phone numbers in unified format for accurate contact.

VoIP services Apps like Skype and WhatsApp use E.164 format to enable international calling.

SMS delivery Authentication codes and notifications reach E.164-formatted numbers accurately worldwide.

Benefits and considerations

Benefits: Numbers work everywhere globally. Format is simple and understandable. Backward compatible with older phone systems.

Considerations: Number assignment rules differ by country, preventing complete standardization. Number scarcity (especially with smartphone proliferation) is becoming problematic. Privacy concerns exist (numbers reveal country/region).

  • ITU-T — the international body defining E.164
  • VoIP — internet calling using E.164
  • SMS — text messages sent to E.164 numbers
  • Country code — the first part of E.164
  • Communication protocols — technology supporting E.164

Frequently asked questions

Q: How many numbers does E.164 support? A: Theoretically about 2 trillion (15 digits), but actual numbers are limited by per-country assignments.

Q: Are all phone numbers in E.164 format? A: Nearly all. However, older PBX systems (corporate phone switches) may use different formats.

Q: What happens if a number exceeds 15 digits? A: E.164 doesn’t support it. Some countries face new allocation difficulties because of this.

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