Use the Explorer watch face (with the LTE dots in the center) to check your signal strength during a call - and be aware the signal when the call is active may not be the same as when the device is at standby so you need to watch both conditions. If the signal level drops during the callctive cell, so check again in a different location, and call your network to check for system faults or maintenance.
If you can’t see any obvious problems your Series 4 is still in warranty, so you could also Contact Apple Support and arrange to get your watch checked by Apple. If the problem is every call the best check would be a Genius Bar appointment so you can demonstrate the problem face to face.