The external alert port is NOT a dry contact, but instead provides 48VDC for a compatible device. You can't use a standard 90VAC ringer. I'm not familiar with being able to puch a button on a set to trigger the alert manually. Check the manual for that. I suppose you could rig it up to drive a relay directly, which would switch an AC power supply and then a ringer, which you could also parallel a pushbutton switch to the relay contacts for manual signalling.