Unlocking your handset will enable you to use any SIM card in your phone, allowing you to take advantage of more competitive tariffs from rival networks. You will also be able to use SIM cards purchased whilst abroad, allowing you to use local networks and make significant savings on the cost of calls and SMS messages. Finally, unlocking your phone may add value should you wish to sell it. The ability for a new owner to use their SIM card of choice will be far more appealing than having to pay to have the handset unlocked first.