Just because you have WiFi calling and you are on WiFi, doesn't mean you are using WiFi calling. It's my understanding that T-Mobile is the exception, with AT&T and other carriers using WiFi calling only if the cellular signal is non-existent or very weak. That would explain why you get dropped calls.
I second this. When Wi-Fi calling first came out on ATT, I had been using a Micro Cell with mixed results. My ATT signal was 0-4 bars in my home. I had very few issues with MicroCell because my office (where I take most cell calls) had 0-1 bars and relied on Microcell. My wife usually took calls in an area of the home where the signal ranged from 2-4 bars and had a lot of one-way, dropped, garbled calls. Wi-Fi calling worked similar with her having mixed results and mine working pretty well. As it turns out, ATT "favors" using their LTE network and will switch to it whenever there are 3+ bars. The handoff is not without issues, so fluctuations in the signal were problematic. Even in the best reception areas, we saw 2-4 bars, the tower sat at the top of a hill next to the Interstate and roaming 3G\LTE users on the freeway during heavy traffic caused a lot of signal fluctuations in our home (confirmed by their engineers).
After a few months of this, we switched to T-Mobile and Wi-Fi calling is always active in all locations in our home, Wi-Fi is pretty good throughout the home. We never have issues anymore unless we turn off Wi-Fi on our phones.
So, if your LTE\3G signal is consistently less than 3 bars, both should work pretty well, but I think you have to go all-in with one and disable the other. I found myself having to reboot the Micro Cell pretty often, not so with Wi-Fi. Finally, if Wi-Fi calling is enabled, it will work on any Wi-Fi you join, so at work my signal is 1-2 bars and Wi-Fi calling is excellent, as it is in friends homes and other Wi-Fi I commonly use. Since Wi-Fi calling is an on\off setting, using it results in better calling experiences in more places than relying on Micro Cell at home.
But, as long as Ma Bell insists on favoring their LTE network, both solutions will be problematic. I suggest looking at another carrier.