phone switches from LTE to 3G as soon as calling someone and stays on 3G, why?

My phone (iphone2s) switches from LTE to 3G as soon as I start calling someone and stays on 3G. The only i have figured is to go to setting -> cellular -> cellular data option ->Voice and Data and then switch from LTE to 3G and then from 3G to LTE again. This is weird.

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