Sprint Nextel wants to make sure its iPhone 4S customers are going to stick with it.

The wireless carrier plans to begin locking the SIM on iPhone4S devices today, as first reported by SprintFeed. The company will only unlock the SIM when the phone is taken abroad.

Sprint confirmed that the SIM is locked for both domestic and international calling for the iPhone 4S. Customers in "good standing" for at least 90 days can request to have their SIM unlocked for international use.

With the SIM locked down, customers are unable to switch carriers domestically. For years, AT&T iPhone customers would jailbreak their phones and switch carriers. T-Mobile, for instance, offers a Micro SIM cardfor customers who want to bring their iPhones to its network.

It's easy to see why Sprint would be protective of its iPhone 4S base. The company reportedly committed a hefty $20 billion in iPhone orders over the next four years. In order to ensure a solid return on its investment, it needs its iPhone customers to stay with Sprint.

The locking procedure occurs during the activation process "and is invisible to the customer," SprintFeed reported. Interestingly, SprintFeed said the initial run of iPhone 4S devices shipped from Sprint came with an unlocked SIM, and those phones won't be affected by the new policy.