Knowledge Base article #306435 addresses an issue where magnetic cases may cause inteference with wireless signals transmitted by the iPhone. The document states:
"Remove the phone from the carrying case. Turn iPhone off and then on again. Press and hold the Sleep/Wake button on top of iPhone for a few seconds until a red slider appears, and then slide the slider. Then press and hold the Sleep/Wake button until the Apple logo appears. Use iPhone to verify the issue is no longer present."