The actual MCC and MNC assignments are ITU T E.212 and the (U)SIM Header of the ICCID is
ITU T E.118 document.
And as a side note, the (U)SIM Header is between 6 and 7 digits. The MNC is between 2 and
So in theory E212 should be enough. Each operator (MVNO or otherwise)
should have its own MCC/MNC identifier. However, this practice came in
too late to the game, so this is not true in reality.
Many operators assigned MVNO SIMs out of their pool, resulting in chaos.
Hence why DBs resort to playing tricks with EFspn, EFgid1, etc.
I suspect newly issues SIMs do not have this problem, but it might still
be relevant for SIMs issued in the past.