SLMA24 does not provide its own ringing so there must be a ringing module, older hardware was the RG card but then it switched to the RGMOD, a bit bigger than a cigarette packet and gold or silver coloured. RGMOD is screwed to the backplane of the shelf.
If only one SLMA24 has this issue I don't see how it can be the ring gen. Like sbcsu says, I would swap the SLMA24 with another one, preferably from another shelf, just take them out and swap then over, downtime it just a few seconds. If the problem moves with the card, you could maybe try replacing the loadware on the HD just to flash new software onto the card, or you may need a new card. If it stays on the shelf, ie problem appears on the same PEN which is now a different card, then maybe it's the RG although I would expect all SLMA24 to be affected, or maybe there is some overload going on, which would show in hista.
You can exchange the RG with one from another shelf but the shelf must be powered off.
I am assuming here that when you call the phone - A party hears ringing, B party does not ring (no audible ringing) but if B party goes off hook, the call is connected. That is a ring gen problem.