IMEI and IMEI-SV – International Mobile Station Equipment Identity and Software Version Number is a number used to unique identify a terminal (mobile phone) on 3GPP networks and satellite.
IMEI
As we said all mobile devices must have a IMEI number associated with each
To verify your IMEI press *06# on your mobile phone.
If your mobile phone was stolen report to your ISP and share the IMEI with them to blacklist the phone.

IMEI Number Structure
IMEI numbers have 15 digits but only 14 are used on telecom the other digit is used no satellite connections. The number is divided into three sections defining the Company

TAC – Type Allocation Code
Serial Number
Check Digit
IMEI-SV

Mobile equipment identifier (MEID)
Technical Specification Group Core Network and Terminals; Numbering, addressing and identification
Permanent Equipment Identifier (PEI)
https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=729