Re: HD Voice, AT&T & The 950
I find that the phone loses its VoLTE registration randomly, and it can take a few tries to fix. Here are some things to do, in order of ascending complexity:
1) Reboot the phone
2) Settings --> Network & Wireless --> Cellular & SIM --> Scroll down, SIM Settings (no idea if there's an easier way to get to this). Do you have "Enhanced 4G LTE Services"? If yes, is it registered? Toggle the on/off switch and see if that gets the phone to register.
3) If you don't have "Enhanced 4G LTE Services", then you'll have to power off the phone, take out the SIM, power on, wait a minute or so, then power off. Re-insert the SIM, power on, and with any luck you'll get the "SIM settings have changed" prompt, and then reboot the phone. Double check the settings for Enhanced 4G LTE Services, and you should be registered then.
Hope that helps!