System information type 5 & 6

Hi, i'm capturing my own voice calls in wireshark. i can see a system information type packet just before ciphering command. 204 frames further there is a system information type 6 packet. but these 2 packets don't resamble... how can i subtract a key out of these 2 different packets? this doesn't make any sens.... if i look for the same type 5 packet... they are never at the same location.... :( is my reasoning wrong? kind regards