If you don't have VoLTE turned on (or if the phone doesn't support it on Metro), that's normal, since the phone is reverting to GSM to make the voice call, and it doesn't have dual antennas for both voice calls and mobile data. Go to the Settings>Mobile Networks menu and look for an option for VoLTE (which might also be called Enhanced 4G, Advanced Calling, or HD Calling), and turn it on.
Also, in the specs, I also see that the phone supports both GSM and CDMA. If it's set to prefer CDMA, that might also be a problem. Go to Settings>Mobile Networks>Preferred Network Type and make sure CDMA isn't selected.