The MSISDN can be stored on the SIM.  It's often put there by
the network provider as the first entry in the "own numbers"
phonebook.  But it does not need to be stored anywhere on the
phone or SIM, and does not need to be correct if it is stored
in the above manner.

Some phones allow users to read, edit or delete these "own
number" entries via the phonebook menu.  And they can usually
be read over a serial interface with "AT+CNUM" command.
Likewise they can be deleted and added with "AT+CPBW" after
first setting the phonebook to "own numbers" with the

       AT+CPBS="ON"

command.

The only absolutely reliable DIY method of establishing your
MSISDN is to call someone and look at their calling number
display, or SMS someone and look at the sender's address.

John