How to extract the 15 digit IMEI from 21 digit IMEISV?

Specs & Protocol Testing
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Dear Experts.
How to extract the 15 digit IMEI from 21 digit IMEISV?

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https://support.hologram.io/hc/en-us/articles/360035696673-IMEISV-vs-IMEI

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Are you sure that IMEISV has 21 digits?

I know 16 digits:

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image906×590 100 KB

From 3GPP 23.003 (16 digits):

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image708×422 59.8 KB

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This is what surprised me.
I think beyond 16 is the device defined and not part of standards.

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Check this:

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What is the significance of IMEISV Profile?