The IMEI displayed in the Customer Portal is retrieved from the network during PDP context activation, the format is as follows:
IMEI + SV (software version), based on the standard specification 3GPP TS23.003.

Composition of IMEI:

Type Allocation Code (TAC) - 8 digits

Serial Number (SNR) - 6 digits

Spare digit - 1 digit

Composition of IMEISV:

TAC - 8 digits

SNR - 6 digits

Software Version Number (SVN) - 2 digits

If you want to compute the original IMEI from the IMEISV, you can apply the algorithm of Luhn. The Luhn Check Digit (CD) is computed on the 14 digits after the SVN digits are ignored:
Step 1: Double every second digit, starting from the first one on the most right.
Step 2: Add all digits that have been left out in step 1 and then add each individual digit that you got in step 1 (if you got a 12 in step 1, you need to add 1 and 2).
Step 3: If the sum obtained in step 2 ends in 0, then CD is set to 0. Otherwise, the CD is calculated by subtracting the sum from the closest higher value ending in 0 (if you got 18, then CD=20-18=2).