Taken from the Cyanogen Wiki CyanogenMod Wiki
Most carriers "lock" their handsets to discourage customers from taking their handset and moving to another carrier. Carriers also depend on "exclusivity" agreements to encourage users to switch to their plans so they can use a particular handset (e.g., Apple's iPhone on the AT&T network). To use the handset on another carrier's network it would be necessary to "unlock
" the handset. This is done with a code based on the IMEI of the handset that can be provided by your carrier or firms on the internet that are slightly more reliable than west African ebay bidders. Unlocking is not possible with CyanogenMod or any other Android replacement.
Furthermore, they may impose software limitations to prevent using the handset in a manner that might undercut their voice plans (e.g., Skype, Google Voice), or putting strain on the data network (e.g., tethering
, streaming video). The way to get around this is to acquire 'root' (i.e. Administrator) access on the device, so you can install/modify/fix/break anything you want.