Thanks. I actually solved this problem yesterday afternoon, with help from the experts at Microsoft "tier 3" support. The engineer had me to packet captures on our F5 units for both an iPhone and a WP7 connection. The former fails, the latter succeeds. He compared the packet captures and found that the iPhone tries to initiate a TLS v1.2 connection, and the F5 closes the connection. The WP7 initiates a TLS v1.0 connection, and communication continues. With that information, I found this F5 KB article: https://support.f5.com/kb/en-us/solutions/public/13000/000/sol13037.html?sr=19939546. This article identifies a TLS bug in specific firmware versions when using TLS v1.1 or v1.2. The solution is to upgrade to at least BIG-IP v10.2.2 HF1. I was on the release version of 10.2.2. I updated to 10.2.2 HF4, and iPhones and iPads could suddenly connect fine.