Why does the data service drop to 3G from 4G when voice calls are made?

Depending on which device/ Operating System you have; Go to settings, Mobile settings, Network Mode, Select 4G only instead of 4G/3G .

In the absence of the above CS mechanism for a voice call on a 4G network, LTE networks use a mechanism called Circuit switched Fallback or CSFB . This makes use of the existing 3G and 2G networks and tries to channelize voice communication for 4G, by switching 4G to 3G for a voice call.

When you make a call, your phone detached from 4G to 3G because 4G, unless voice over LTE is activated in the network, can’t facilitate phone calls directly . Your call will have to go back to 3G (it’s called CSFB – circuit switch fall back).29 Ağu 2018

Originally Answered: Why is my data off while I am in a phone call? Because the device is either not capable of VoLTE or the sim card is not provisioned for VoLTE . VoLTE is Voice over LTE.