Have you recently upgraded your Apple IOS ShoreTel Mobility client to ShoreTel Mobility 9, now you have one way audio? This is because ShoreTel have decided to drop support for G729 codec, and your administrator at one point decided that you should be using it.
The upgrade was submitted to the Apple app store on the 4th of September 2015.
The resolution (to be completed by an administrator):
To disable the user from using that codec, go to Mobility Router Admin portal under Configuration->Mobility->Default Settings and disable the codec G.729 Annex-B for all the locations Enterprise, Home and Cell Data.
After updating configuration, users are required to restart their Mobility client to sync the settings.
More details on ShoreTel Mobility 9 one way audio can be found at the ShoreTel KB here