What I meant by my response was, if the device is not shipped from the factory with Google Services installed, it's not supported by 3CX.
In this article, it mentions: "Due to the ongoing restrictions imposed by the US government, Huawei's phones can't use any Google services, including Gmail, Maps and Chrome and, most importantly of all, the Google Play store."
Thank you for your detailed explanation. It might be the issue, even though the phone has google play store. I'll check it out with the phone owner to see if that's the issue.