phone reboots every time I make a call or answer after iOS 11.3 update

I updated my device to the iOS 11.3 and ever since i haven’t been a able to answer or make calls without my iph 7 Plus reboot/restart every time !? I tried hard reset, erase phone from From My iPhone, ITunes update and restore, reset settings and nothing also Siri doesn’t work neither I thought my microphone was the issue but I can hear myself when recording a voice memo check . Please what is there to do? Am I able to downgrade back to the iOS I had before the change