Modern mobile networks use exactly the same protocol to carry voice and data. Because voice is just data. When your call is fading or being intermittent then the packets are being dropped. In such situation packets of your mobile data for instance a web page being loaded by a browser are also being dropped. Mobiles drop packets left and right when reception deteriorates or there are too many subscribers trying to use the shared radio channel. And HTTP2 or 3 can't do much about it because it's not magic, if you lose data you need to retransmit it which TCP and HTTP/1.1 can do just as well. BTM UMTS which you claim you were so professionally involved in also uses converged backbone and carries both data and voice the same way so you should have know it already lol :)