One interesting aspect of the Android 5.1 update that I discovered recently on my Google Nexus 6 by Motorola is that the HD voice icon won’t always show up for all HD calls made on the T-Mobile network. It will only show up for HD calls made over VoLTE – HD calls made over HSPA+ will not have this icon.
How do I know this? My partner and I both have T-Mobile phones – I have the Nexus 6, and he has the Nexus 5. We’ve made plenty of HD calls back and forth to each other over HSPA+. I know what an HD voice call sounds like. And reliably, every single time I call him over HSPA+, it sounds just like an HD call, but there is no icon. HD calls over VoLTE are indicated as such, just like normal.
This is interesting, and I’m unsure if this is just an oversight on Google’s part, or whether it’s a conscious decision to only label calls over LTE as HD-capable. In any case, we’ll see if anything changes in a future OTA update.