Sprint will begin locking all iPhone 4S SIM cards at activation as of today, according to a new report. Citing an internal Sprint memo, Sprintfeed reports that the memo indicates that all iPhone 4S units will have the SIM locked beginning today. “The locking occurs during the activation process and is invisible to the customer,” the memo reads. “The process for unlocking a SIM is the same as enabling international GSM roaming. [Representatives] should direct the customer to Sprint Worldwide for authentication.” The memo goes on to state that customers can sign up for an international rate plan and use the phone in most countries around the world, and that the update will not impact any current iPhone 4S customers who activated their phones prior to today.