A MSL (Master Subsidiary Lock) code is like an administrator password to your phone. Carriers like Sprint set a lock on certain settings to prevent users from messing up or changing these phone-critical settings, usually settings that relate to carrier specific networking.
Usually the only way to get your MSL code is to call your carrier and ask for it. Sometimes they are willing to give it out; other times they will not give it out unless there’s a reason for it, such as to reset your modem.
MSL codes are also unique to your phone. Your MSL code will be different from your friend’s MSL code, even if you both have the same phone on the same carrier.
To get your MSL code on your Nexus S 4G without calling Sprint, open up an ADB terminal and enter the following:
shell@android:/ # getprop ril.MSL
getprop ril.MSL
000000
shell@android:/ #
In this case, “000000” would be my MSL code.
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