There seems to be a few videos on YouTube that address call handling with Mi branded smart bands. One from last year shows how to set up a host app on your device that may give you a bit more functionality but the article in the link appears to say that calls cannot be answered via the device due to the device not having a speaker. This would be why it vibrates when you receive calls, and simply works as a notification device. You can see the article here...
https://forums.androidcentral.com/e...calls-on-xiaomi-mi-band-4-8867&token=oQzW0fJp
I think browsing some of the videos available on YouTube would yield better answers to your question than we could as the odds of you finding someone with the device in a sea of Android users, out of the few that visit this one site, is likely to be very low. This is not to say that we won't try. Use mustang7757's link to offer more details after reading the link article I provided to see if it fits your issue and we will do our best.