HTC One S on T-Mobile is reported to suffer battery drainage during WiFi calls. Customers have reported that the unique feature of T-Mobile’s WiFi calling is draining the battery unusually fast.
T-Mobile said that a fix for the reported issue will be coming soon in an update. For now T-Mobile advises users to keep the phone on charging while making WiFi calls. The permanent solution will be coming in a future update.
There were issues reported with the HTC One X that were fixed immediately via an over-the-air update from HTC. We can now expect the same for T-Mobile HTC One S that will fix the reported issue.