Have a OS 7200 and the SLI card is not making a audible ring, ie. you call an SLI extension and the handset doesn't ring, however you can pick it up and the call will connect.

SLIs can ring out OK, and work normally just no ring generated on them when they are dialled internally or externally.

I had this issue years ago with a DCS and it was a bad RGEN unit in the KSU, not sure that the OfficeServ 7xxx had anything like this? The DCS500 still has an RGEN card.

There is an rgu for the os7x00. What sort of sli card do you have? It's it one of the newer sli3 cards?

Older style 8SLI... I assume there should be a RGU in the J3 connector next to the PSU?

You are correct.
The rgu had been passed out due to the new sli3 cards having it built in.
Add such all new os7200 and os7400 ksu's no longer come with rgu's

You must connect external ringer card. You have to open back cover n its beside the PSU.