Glossary

MSISDN, IMSI and ICCID: telecom identifiers explained

A single SIM carries three distinct identifiers that are often confused: the MSISDN is the dialable phone number itself, the IMSI is the subscriber identity the carrier's network uses internally, and the ICCID is the serial number of the SIM card. They are not interchangeable, and most phone lookup APIs, Telebase included, only ever deal with the first one.

MSISDN: the dialable number

MSISDN stands for Mobile Station International Subscriber Directory Number. In practice, it is simply the phone number as you would dial it, written in international format such as +447700900000. This is the identifier a phone lookup API accepts as input and returns in its response, because it is the only one of the three that a third party is ever meant to see or handle.

IMSI: the network's internal subscriber identity

IMSI stands for International Mobile Subscriber Identity. It is a number stored on the SIM that the carrier's own network uses to authenticate the subscriber and route calls, texts and data. It is not the same as the phone number, and a carrier can technically reassign which MSISDN maps to a given IMSI, which is part of what makes number portability possible. Carriers treat the IMSI as sensitive network infrastructure and do not expose it through public or third-party APIs.

ICCID: the SIM card's own serial number

ICCID stands for Integrated Circuit Card Identifier. It identifies the physical or virtual SIM itself, independently of which subscriber or number is currently assigned to it. A subscriber can move their number to a new SIM, for example after upgrading to an eSIM, which changes the ICCID while the MSISDN and the underlying account stay the same.

IMSI vs ICCID: what's the difference

Both live on the SIM, which is why they are easy to conflate, but they identify different things. The IMSI identifies the subscriber and is used by the carrier's network for authentication and routing. The ICCID identifies the SIM card itself, independently of which subscriber or number is currently assigned to it. Neither is exposed to third-party lookup APIs: carriers treat both as network-internal identifiers.

A fourth identifier that is often confused with these three: IMEI

IMEI stands for International Mobile Equipment Identity. Unlike MSISDN, IMSI and ICCID, which all relate to the subscriber's SIM, the IMEI identifies the physical handset itself and stays with the device even if the SIM is moved elsewhere. A phone lookup API does not use or return the IMEI, since it queries the network for information about the number, not the handset.

Why Telebase only works with the MSISDN

Carriers deliberately do not expose IMSI or ICCID to third-party lookup services, since both are used for network-level authentication and exposing them would be a security risk. A carrier lookup API like Telebase accepts an E.164 phone number as input and returns carrier, country, number type and active status inferred from that number, without ever touching the network-internal identifiers. This is the same MSISDN a traditional HLR lookup queries against, since the Home Location Register indexes subscribers by MSISDN, not by IMSI or ICCID. See E.164 formatting and URL encoding for how to format the MSISDN correctly before querying.