Mobile phone locking and unlocking
Mobile handsets are sometimes 'locked' to the network from which the handset is purchased. This means the handset will usually only work when used with that particular provider.
If you want to switch to a different provider but keep your existing handset, you may need to get it unlocked.
You may also need to get your handset unlocked if you want to use a local SIM card in your handset when travelling abroad to avoid roaming charges.