Let first understand what are these
ICCID is a card identifier (physically) where as IMSI is the subscriber identifier. Details about ICCID -
The ICCID (Integrated Circuit Card Identifier) identifies each SIM internationally. A full ICCID is 19 or 20 characters. It is possible to extract the ICCID by using the 'AT!ICCID?' modem command.
The format of the ICCID is: MMCC IINN NNNN NNNN NN C x
MM = Constant (ISO 7812 Major Industry Identifier, = 89 for "Telecommunications administrations and private operating agencies")
CC = Country Code (i.e. 61 = Australia, 86 = China)
II = Issuer Identifier (AAPT = 14, EZI-PhoneCard = 88, Hutchison = 06, Optus = 02/12/21/23, Telstra = 01, Telstra Business = 00/61/62, Vodafone = 03)
N{12} = Account ID ("SIM number")
C = Checksum calculated from the other 19 digits using the Luhn algorithm.
x = An extra 20th digit is returned by the 'AT!ICCID?' command, but doesn't seem to be an official part of the ICCID.
Where as IMSI is the IMSI = International Mobile Subscriber Identity. This is a unique identifier that defines a subscriber in the wireless world, including the country and mobile network to which the subscriber belongs. It has the format MCC-MNC-MSIN.
Now coming to your question - suppose you loose your phone/sim card and operator operator can assign the same old IMSI to your new SIM card which means the mapping is not unique hence you can not now the IMSI by just looking at ICICD number.
