You can put a DR 5 or DR 5.1 software into your cabinet and it will integrate a flash voice mail, but to be fully voice mail compatible, you should be using a DS version cabinet with DS software. Without Disconnect Supervision it is possible that calls answered by the the voice mail will not disconnect when the remote party hangs up.
IMHO it is a mistake to upgrade a non DS cabinet. You would be better served to upgrade the cabinet to a MICS or CICS and reuse your phones. As Dave said you could replace your phones with T7316's when the need arises .
