Which network do you currently have? Verizon?
Unless you have a carrier that has VoLTE (Voice over LTE) then 4G LTE is only used by the phone for data. If you make a call, the cell radio will drop to 3G, because that's the network used for voice. In your case, it may be that you don't have a good 3G signal when you make a call (despite having a good LTE signal), so it's dropping down to 2G (which is what 1X is). You don't really need a superfast network to give decent call quality, so I wouldn't expect it to be bad on 1X--unless the signal is really borderline, and the phone keeps trying to switch between networks.
The Note 5 on Verizon wouldn't do this if you're in an area with their VoLTE (which they call "Advanced Calling")--it would be able to stay on LTE for both calling and data.