International Mobile Subscriber Identity (IMSI) is a unique identifier for mobile subscribers throughout the world.

It’s a one-of-a-kind number used by Mobile Network Operators (MNOs) to identify individual users, and it’s an important part of a SIM profile.

A global system for mobile communications (GSM) and Universal Mobile Telecommunications System (UMTS) network mobile phone user’s international mobile subscriber identification (IMSI) is a unique number, generally fifteen digits.

A GSM subscriber’s IMSI is a unique number.