It's not just on the telcos, Android fragmentation as a whole makes implementing new network features an uphill battle. VoLTE is an absolute mess (on Android) if your build doesn't have settings for your carrier. Google doesn't sell the Pixel in my country, so the only way to enable VoLTE here is to root your device. This is only going to get worse with carriers sunsetting 3G networks - anyone roaming on a Pixel in such a country won't be able to make calls, even to emergency numbers.