E.164 is the ITU-T international public numbering plan. A number is a country code (1–3 digits), a national destination code and a subscriber number, up to 15 digits total, written internationally with a leading '+'. A mobile number in this plan is the MSISDN — the dialable address that maps to a subscriber inside the home network.
What E.164 defines
E.164 is the ITU-T recommendation that defines the international public telecommunication numbering plan — the globally unique format every phone number resolves to. It fixes three things: the maximum length of an international number at 15 digits (excluding the international prefix or '+'), the assignment of country codes to countries and global services, and the structure of the national number that follows the country code. Every dialable telephone number in the world — fixed, mobile, toll-free, premium — is an E.164 number once written in full international form, which is why messaging and voice platforms normalise to E.164 as the canonical address.
The three parts of an E.164 number
An international E.164 number decomposes into a Country Code (CC), a National Destination Code (NDC), and a Subscriber Number (SN). The country code is 1 to 3 digits and is assigned by the ITU (for example 1 for the North American Numbering Plan, 44 for the United Kingdom, 961 for Lebanon). The NDC identifies a destination network or geographic area within the country — for mobiles it selects the mobile operator or number range. The subscriber number identifies the specific line within that NDC. The NDC and SN together form the National (Significant) Number, and CC + NDC + SN must not exceed 15 digits.
| Field | Length | Assigned by | Purpose |
|---|---|---|---|
| Country Code (CC) | 1–3 digits | ITU-T | Identifies the country or global service |
| National Destination Code (NDC) | Variable | National regulator | Selects network / geographic area within the country |
| Subscriber Number (SN) | Variable | Operator | Identifies the line within the NDC |
| National (Significant) Number | NDC + SN | — | The number dialled within the country (with trunk prefix) |
| International number | CC + NDC + SN | — | Globally unique, ≤ 15 digits, written with '+' |
What an MSISDN is
For a mobile subscriber the E.164 number is the MSISDN — Mobile Station International Subscriber Directory Number, defined in 3GPP TS 23.003. It is the number a caller dials or a message is addressed to, and it follows the same CC + NDC + SN decomposition, where the NDC identifies the home mobile network. The MSISDN is deliberately distinct from the IMSI: the MSISDN is the public, dialable directory number, while the IMSI (E.212) is the internal identity stored on the SIM. The HLR/HSS holds the mapping between the two, so a call to an MSISDN can be resolved to the subscriber's IMSI and current location.
+44 7700 900123
│ │ │
│ │ └─ Subscriber Number (SN)
│ └────── National Destination Code (NDC = mobile operator range)
└────────── Country Code (CC = 44, United Kingdom)
MSISDN = CC + NDC + SN (≤ 15 digits, TS 23.003)National vs international format and normalisation
The same number appears in several written forms, and normalising them to one canonical value is a prerequisite for reliable messaging. Nationally a subscriber dials a trunk prefix (often 0) before the NDC; internationally that trunk prefix is dropped and replaced by the country code, prefixed with '+' (the ITU-T notation for 'international prefix'). Normalisation means stripping spaces, hyphens and brackets, removing the national trunk prefix, applying the correct country code for the number's origin, and producing a bare '+CC NDC SN' string. Getting this right prevents mis-delivery, duplicate records and failed lookups; the phone-number formatter tool applies these rules per country.
Input (national) : 07700 900123 (UK, trunk prefix 0)
Strip separators : 07700900123
Remove trunk prefix : 7700900123
Prepend country code : 447700900123
Canonical E.164 : +447700900123E.164 and A2P sender identity
Application-to-person messaging reuses E.164 addressing on the destination side — every recipient MSISDN is an E.164 number — but the originator (sender ID) can take several forms. A long number sender is itself an E.164 MSISDN or a short code within a national plan; an alphanumeric sender ID is a non-dialable text string (up to 11 GSM characters) that is not part of E.164 and cannot receive replies. Because alphanumeric IDs sit outside the numbering plan, their registration and permitted use are governed by each country's regulator, not by ITU-T. Correct E.164 normalisation of the destination, and awareness of which sender types a country allows, together determine deliverability.
| Sender type | In E.164 plan? | Reply-capable | Typical use |
|---|---|---|---|
| Long number (MSISDN) | Yes — full E.164 number | Yes | Two-way A2P, OTP with reply |
| Short code | National plan (not international E.164) | Yes | High-volume domestic A2P |
| Alphanumeric sender ID | No — text string, not dialable | No | One-way branded OTP / alerts |
Frequently asked
What is E.164?
E.164 is the ITU-T recommendation defining the international public telephone numbering plan. It sets the 15-digit maximum length, assigns country codes, and structures every number as a country code, national destination code and subscriber number, written internationally with a leading '+'. It is the canonical format messaging and voice platforms normalise to.
What is the maximum length of an E.164 number?
Fifteen digits, excluding the international prefix or '+'. That 15-digit limit covers the country code plus the national (significant) number — the national destination code and subscriber number together. Many numbers are shorter; none in the public plan exceed 15 digits.
What is the difference between an MSISDN and an IMSI?
The MSISDN is the public, dialable E.164 number a caller or message is addressed to. The IMSI (E.212) is the internal subscriber identity stored on the SIM and never dialled. The HLR/HSS holds the mapping, so a call to an MSISDN is resolved to the subscriber's IMSI and location.
How do I normalise a phone number to E.164?
Strip spaces, hyphens and brackets, remove the national trunk prefix (often a leading 0), apply the correct country code for the number's origin, and produce a bare '+CC NDC SN' string of at most 15 digits. This canonical form prevents mis-delivery, duplicate records and failed lookups.
Is an alphanumeric sender ID an E.164 number?
No. An alphanumeric sender ID is a text string of up to 11 GSM characters, not a dialable number, so it sits outside the E.164 plan and cannot receive replies. Only long numbers (MSISDNs) and short codes are numeric; alphanumeric ID use is governed by each country's regulator.
What is a country calling code?
The one-to-three-digit prefix the ITU assigns to a country or global service at the start of an international E.164 number — for example 1 for the North American Numbering Plan, 44 for the United Kingdom. It selects the destination country before the national destination code narrows it to a network or area.
Source specifications
The published standards this page is grounded in. Opens in a new tab.