Why use reverse nibbled ​in decimal-octets in SMS PDUs?

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I am studying for SMS PDU.

However, I do not know why the SMS PDU transmits the phone number and the like by reverse nibbled. (012345 to 103254)

Why does it work like this?

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Marvin On BEST ANSWER

Noone knows excactly :) But in telco within SS7 protocol, almost all numbers are reversed (vlr,hlr,imsi etc.) In older days on slow networking, every bit matters, thats why SS7 is so optimized. I believe it has to do with a speed when number has odd length. In newer protocols (like Volte) there is no such thing anymore.