T-Mobile's HTC One S hit with battery drain issue while using Wi-Fi calling; software fix is coming
will be addressed in an upcoming software update," although no specific date for the update was mentioned. Calling over Wi-Fi is a feature that could save you the money that you would have had to pay from going over the voice cap on your plan. Having it drain your battery, on the other hand, is not exactly what HTC One S users had in mind when they bought the device. Until the software update is released, following T-Mobile's advice and keeping the phone plugged in while you use the Wi-Fi calling feature should help prevent you from walking around with a dead battery in your HTC One S.
source: T-Mobile via AndroidCentral