I don't know if the 950XL supports this feature. I have an AT&T branded 950. When my phone got the prompt that the SIM Settings had changed, I noticed that I had lost HD voice. I noticed this when my son called me (he's on AT&T with an IPhone 6s) and I could tell that the HD calling was not on as HD versus normal calling is a very distinct difference. So, I checked the settings and noticed that it was gone from my phone. I got to thinking about what had changed since this feature had been working and remembered that I had gotten that message about SIM settings. So, I removed the SIM, rebooted the phone, powered down and reinserted the SIM to see if that would force the settings to update. It did, and the setting reappeared and HD calling is working again. I've not had this problem since. Hope this helps.
Thanks!!! I can confirm that this worked on my AT&T 950. The steps I took were slightly different, although not much. I shut the phone off, pulled out the SIM, and turned the phone back on letting it boot all the way. Then I shut it off again. Re-inserted the SIM and turned it back on. I went to the SIM Settings and I was immediately prompted with the message "Restart Required. Your Phone's settings or the SIM card have changed. Restart your phone to complete the installation." At this time, the HD Voice setting was still not there. However, after rebooting again as the message suggested, I checked the SIM Settings and the option was available (and also enabled automatically). Thanks so much for your help VineRider.
As a side note - I really like this phone but damn is it buggy. I hope Microsoft really nails down some of the stability issues.