# What is an MSISDN?

> MSISDN (Mobile Subscriber ISDN Number) is the technical name for a phone number — the E.164-formatted number (country code plus national subscriber number, up to 15 digits) used to route calls and SMS to a subscriber. It is the value you put in the `to` field of an SMS API call. The MSISDN is distinct from the IMSI, the internal SIM identifier the network uses on the signalling plane and never exposes to other subscribers.

- Canonical URL: https://www.broadnet.me/answers/what-is-msisdn
- Language: en
- Last reviewed: 2026-07-22
- Publisher: BroadNet (https://www.broadnet.me)

## MSISDN vs IMSI vs ICCID

Three identifiers are easy to confuse. The MSISDN is the public, dialable routing number. The IMSI is the network's internal subscriber identity, stored on the SIM. The ICCID is the SIM card's own serial number. A subscriber can change SIM (new ICCID) or keep their number when switching operator (MSISDN preserved via MNP). See /answers/what-is-imsi.

## Formatting for A2P: always E.164

Pass MSISDNs in strict E.164 form — a leading +, the country code, then the national number with no spaces, dashes, or national trunk zeros. Local formats (a leading 00, or a national 0) are the most common cause of routing failures. Validate and normalise with HLR Lookup before sending. See /answers/what-is-e164 and /products/hlr-lookup.

## Related

- [All answers](https://www.broadnet.me/answers)
- [Glossary](https://www.broadnet.me/answers)
- [What is an IMSI?](https://www.broadnet.me/answers/what-is-imsi)
- [What is E.164?](https://www.broadnet.me/answers/what-is-e164)
- [What is MNP (Mobile Number Portability)?](https://www.broadnet.me/answers/what-is-mnp)
- [What is HLR Lookup?](https://www.broadnet.me/answers/what-is-hlr-lookup)
- [SMS API](https://www.broadnet.me/products/sms-api)
- [HLR Lookup](https://www.broadnet.me/products/hlr-lookup)