nawk -v "in_msc=923210002002" '
   /^</ {
      split($0, a, "=");
      split(a[2], b, ",");
      msisdn=b[1];
   }
   /^VLR/ {
      getline;
      msc=$2;

      if (msc == in_msc) { print(msisdn); }
   }
' input_file.txt