Things change. Verizon is one I saw most of the time while roaming in the US. T-Mobile popped up initially but it didn't work well so the phone seemed to stick with Verizon after that.
Somehow I don't think that'll be the issue with many current phones. Especially the iPhone, which I was using. T-Mobile seemed to have weaker signal on the Vegas strip (even outdoors and outside the T-Mobile store) that kept dropping to 3G. After that the iPhone switched to Verizon and stayed there.
It most certainly is the issue. When your TELUS phone doesn't have the ability to use VoLTE, it falls back to 3G, but this is a network that Verizon does not have. AT&T and T-mobile do. So when Telus moved from CDMA to the GSM standards, they had to change roaming partners for those phones - to partners like T-Mobile and AT&T. Since Verizon migrated later in the game, their LTE does not have the fallback to the correct 3G....now that VoLTE roaming has come into play, it is compatible with the current Verizon network.