Following Apple’s release of iPhone OS 3.1.2, AT&T has released a carrier settings update for its U.S. iPhone customers. Listed as AT&T 5.6, it is unclear what improvements the new settings file may offer; the last carrier settings update activated MMS service for U.S. iPhone customers. AT&T announced earlier this week that it will now allow VoIP iPhone applications to operate over its 3G network, but it is unknown whether this new update is in any way related to that announcement.