The only North American carrier that will permit a GSM device to work on the LTE networks without attempting to drop voice calls to 3G (by adding a "CDMAless" note to your account) is Verizon. Most carriers, like the four national Canadian carriers who serve my area, will tell you as they told me: "nope, we won't do that. If you don't want your voice calls to drop to 3G then you need to buy a new device from us". It would just take a few key strokes to add the IMEI of your device and a note that it is a CDMAless provisioned GSM phone to your account, but they won't because they make money selling their own phones. Some carriers will provision VoLTE on 'certified devices' not purchased from them, but here is the catch - they charge a large fee to phone manufacturers to have their devices listed as "certified" on the carrier's network (not that any actual testing is done), and companies like Xiaomi, OnePlus, Oppo, and Vivo won't pay that fee to carriers in regions where they sell only a few phones. In contrast, they have paid the British and most European carriers the certification fee because they sell lots of phones over there. Most Hong Kong mobile phone stores do a thriving business in Britain going back to the days twenty years ago when Hong Kong was still a British colony, and TradingShenzhen, based in mainland China, is a German company that has made duty and tax free imports of Chinese phones into Europe very popular there. I guess if you live in Nova Scotia or Arizona Vivo products won't be 'certified' by local carriers. I have a Huawei Mate 20 X that actually is 'certified' by my carrier and works with VoLTE and WiFi calling, but only because Huawei used to sell a lot of phones here until the government banned Google from working with them and most North Americans have become so Google and Android dependent that they don't want Huawei's new alternative Harmony OS operating system. I did try it out on my Mate 20 X before realizing that without Google Services I couldn't call an Uber or have Gmail update my calendar with important events. I re-flashed Android 12 on the device and I'm still getting monthly patches and security updates. Sorry for the long winded explanation.