Nevertheless, the fundamental principle of GSM and its successors, is open interfaces which encourage competition among multiple vendors. This is the reason a mobile phone is, in fact, a combination of phone and the subscriber identity module(SIM). Locking the phone to a network is not much different from having the SIM built into the mobile phone. Network operators in many industrialized countries are not bound by law to give the phone unlocking code to subscribers even after the expiry of the contract period. Mobile phones with multiple SIM cards are quite common in India. Most phones sold in the UK are network locked and single SIM but SIM-free phones are available.