#!/bin/bash
s='"ID","Name","DOB","Age","Address","City","State","Country","Phone number"
"123","ABC","12/20/2020","0","No.38,3rd st, RRR NNN, TRT",,,,"9999999999"'
perl -pe 's/"([^"]*)"|,/defined($1) ? $1 : "|"/ge' <<< "$s"