When you say VOIP it sounds like you mean VOIP trunks rather than endpoints (telephones, etc). If you are using a full PRI moving to IP trunks could be done with little on your side. Multiple companies offer IP trunks that look like CO lines or PRIs to the telephone system. Our experience has been mixed. Some sound OK most of the time, others sound fine. You would need a fair amount of bandwith coming in though...what is your plan for that? Going from a full PRI to 23 IP trunks is a big commitment.

Honestly though, cost savings does not seem to be too significant. Are you looking for LD savings or what?If you really desire IP look into a system that supports IP trunks/endpoints rather than rigging something that was never meant to have it.