Two suggestions I have for you to try to see if they make a difference:I have full AC 5Ghz coverage throughout my property (70'x100' lot with 1,750 sq ft house). I have had dropped calls when on cellular with my wifi signal strong (AT&T's transition from cellular to wifi issue or vise versa). When forcing wifi calling I've had issues the other side said I cut in and out, but I hear them clearly. This has gotten worse since iOS 11, but was happening even prior on iOS10 (on multiple phones).
AT&T's execution of wifi calling is a mess...
1) Turn off Wi-fi Assist - that may be getting into a race condition with Wifi calling. (It's in cellular options)
2) Under Cellular, go into Cellular Data Options, then Enable LTE - if your phone is set to Voice & Data, try setting it to just data.
The second option will be disabling VoLTE. I was just bouncing around a bunch of forums - it appears the threshold for switching from Cellular to Wifi for Voice may have been set lower for 4G than 3G. That may not be giving the phone enough time to switch the context so it doesn't drop the call.
I think it's entirely possible that with iOS 11 a lot of the carriers may have taken the option away in their carrier settings (or maybe Apple did, to simplify support). T-Mobile certainly had the option before - we should get someone that's on T-Mo to take a look and see if the setting is still there. (Prefer wifi calling)