GSM Smartphones:
The International Mobile Equipment Identifier (IMEI) is a unique 15-digit number that identifies a GSM cellular phone handset on its network. This number is generally found under the battery of the cellular phone. The first eight digits are a Type Allocation Code (TAC), and the next six digits are the Device Serial Number (DSN). The final digit is a check digit, usually set to 0.

The Mobile Station International Subscriber Directory Number (MSISDN) is the phone's 15-digit, globally unique number. The MSISDN follows the International Telecommunication Union (ITU) Recommendation E.164 telephone numbering plan, composed of a 1–3-digit country code, followed by a country-specific number. In North America, the first digit is a 1, followed by a 3-digit area code.

The following links are good resources for researching IMEIs:
https://www.imei.info
https://www.numberingplans.com/

CDMA Smartphones:
The Electronic Serial Number (ESN) is located under the battery of the cellular phone. This is a unique 32-bit number assigned to each mobile phone on the network. The ESN may be listed in decimal (11 digits) and/or hexadecimal (8 Hex digits). Be aware that the Hex version of the ESN is not a direct numeric conversion of the decimal value.  An ESN converter can be found at https://www.elfqrin.com/esndhconv.php.

The Mobile Equipment ID (MEID), also found under the battery cover, is a 56-bit number that replaced the ESN due to the limited number of 32-bit ESN numbers. The MEID is listed in Hex, where the first byte is a regional code, the next three bytes are a manufacturer code, and the remaining three bytes are a manufacturer-assigned serial number. CDMA phones also have two other identifying numbers, namely the Mobile Identification Number (MIN) and Mobile Directory Number (MDN). The MIN is a carrier-assigned, carrier-unique 24-bit (10-digit) telephone number. When a call is placed, the phone sends the ESN and MIN to the local tower. The MDN is the globally unique telephone number of the phone. Prior to Wireless Number Portability, the MIN and MDN were the same, but in today's environment, the customer can keep her phone number (MDN) even if she changes carriers.

Reference:
https://for585.com/3jas7 (Phonescoop.com network definitions)