Bill's right. #2 will work best, especially since you already have the ring generator. You are right about the 2500's though; you can't run ringing across Tip and Ring for the intercom. You'll need to continue using the second pair and just wire it to the ringer in the set instead. The connections are as follows:

Black: Goes to black ringer lead on any blind terminal;
Yellow: Goes to red ringer lead on any blind terminal;

Make sure that the slate/red and slate ringer leads (if present) are connected to terminals A and K on the phone's network.

There's nothing wrong with connecting ring voltage to the RN and RG inputs on a Tone Commander intercom; they are made to permit this.

Also, remember that tip and ring of all intercom stations come back to a common tip-ring of the intercom, so if you tried this, a call to one station would have the potential to ring all 2500's connected this way.

As for the loud ringing when the phone is off-hook in use, there is a set of switch hook contacts that can be rewired to disable the ringing on most 2500 sets when off-hook.

Of course, you could keep it simple and buy loud ringers that work on 10-12 volts AC in the loud area.

Ed Vaughn, MBSWWYPBX