I have a customer with a STS and a Valcom 2003a. He has 2 external horns that ring on the CO lines. He wants the volume to be turned down ONLY for the ringing and not to effect the paging and BGM playing. Is this possible?

That depends on how you wired the Valcom unit. It has separate volume controls for page and tones. There is no volume control for the ring tones that come out of the ext page port built into the STS.