HTC and T-Mobile have a fix in the works to address the battery drain issues while the HTC One S is using Wifi calling. There's no real detail about what the issue is, but AC member venious, who has been using the One S since Day 1, thinks that while on a call using the Wifi calling feature, the battery is drained more than normal. So far, battery life has not been a major concern across the various Internet forums for the device.
Wifi calling is a big draw for many (including yours truly) and it's great to see any issues identified so quickly. While we have no word on when to expect any software patch, this being one of HTC's (and T-Mobile's) flagship phones means they should be prioritizing any fix. We'll all keep an eye out for it. In the meantime, we're talking all about the things we love, the things we don't, and the lust we have for this one in the One S forums -- jump in and join us!