I am a Huawei User in the US.
Here is the Issue:
ATT and T-Mobile Announced that they planning to shutdown UMTS networks. So VoLTE will be required for all devices
First. Does your signal drops to 3G during a call?
If signal stays on LTE you will be fine because VoLTE is working
If not there is the issue. You will require VoLTE. AT&T have a very strict policy about not allowing BYOD devices for VoLTE.
But also Huawei is not offering VoLTE support for US SIM Cards. Even if the devices support US LTE Frequencies. This is Huawei is not taking care of. Many unlocked devices from different brands offer VoLTE support when used in ATT or T-Mobile US
I have a Mate 20 with T-Mobile USA. The device doesn't support VoLTE because Huawei needs to add just a software update for VoLTE with T-Mobile and ATT. There is plenty of users with the same issue. Some models like P20, P20 lite and P30 support VoLTE with at least T-Mobile. I had also a Huawei P20 Lite and VoLTE worked with Cricket and ATT if VoLTE was provisioned in the line
Of course the carrier needs to offer VoLTE but also Huawei needs to offer VoLTE support with US SIM Cards. And I been trying to escalate in Huawei Tech support but the service is very bad. They don't seem to care.
If VoLTE is not working in your device. You will be not able to make phone calls when UMTS network shuts down:
I Just made a post on Huawei Forums for see if we can report the issue. The more the people reports that increases the chances that this can be fixed:
https://consumer.huawei.com/en/community/details/topicId_120349/