Netherlands                               MCC        MNC
Tele2 (Netherlands) B.V.       204          02
Vodafone Libertel N.V.          204          04
KPN Telecom B.V.                    204          08
BT Ignite Nederland B.V.       204          12
BEN Nederland B.V.                204           16
Dutchtone N.V.                         204           20
NS Railinfrabeheer B.V.         204           21

AT+COPS? to obtain MCC and MNC

AT#CSURV?
AT#CSURV
Network survey started…
arfcn: 48 bsic: 24 rxLev: -52 ber: 0.00 mcc: 610 mnc: 1
lac: 33281 cellId: 3648 cellStatus: CELL_SUITABLE
numArfcn: 2 arfcn: 30 48 numChannels: 5 array: 14 19 22
48 82
arfcn: 14 rxLev: 8
Network survey ended
OK

AT+CREG? to obtain LAC and CID
Command AT+CGREG?  gives answer +CGREG: <n>,<stat>[,<lac>,<ci>]  or if not available +CME ERROR: <err>

You can also get LAC and CID codes by downloading a software on your mobile phone:
Cellspotting GSM locator:              http://www.cellspotting.com      – Nokia S60 or above, Sony Ericsson JP7 or above
Network Monitor GSM locator:    http://www.ottaky.com/midlets.php       – Sony Ericsson JP7 or above
Cell track: http://www.afischer-online.de/sos/celltrack/      – Symbian S60 or above
Net Monitor GSM locator:             http://www.androlib.com/android.application.com-parizene-netmonitor-iCEn.aspx     – Android
Cell finder GSM locdator:               http://code.google.com/p/codetastrophe/     – Android
Cell Tower Locator GSM locator:  http://www.appbrain.com/app/cell-tower-locator/tm.app.cellTowerLocator    – Android
Install the app on your phone. Start the App and get the LAC and CID codes.

You try to get as many as you can because the more you have the more accurate will be the location.
The concept is that the further away from the transmitter the weaker will be the signal. So you can draw circles on a map around each transmitter location describing the strength of the received signal.
If you have two transmitters received at your GSM location you draw for example the signal strength 5 circle for one and the strength 3 circle for the other. Now you probably have two points where the circles cross. A third antenna can definitively give the location by simple triangulation.