is the network remain same ? which mean 3g full bar to 3g 1 bar when u receiving a call ?

since normally our phone stay @ 3g/4g while using data, but sometimes while making call, they would use another network,
as my experience, when i set my phone network to LTE ONLY, which means there would be no network at all if LTE is not available, i can't make / receive calls/sms even i am able to use data of LTE, since u mobile does not support voLTE, they need 2g/3g for phone calls

Is there anyone know why iPhone 6S always shown 3G only while my friend iPhone 5S always maintain LTE connection. We both at the same place and time.