I *think* the Revert N feature holds the unit "open" for N seconds after loss of carrier rather than muting as soon as carrier is lost. I've slept since I last set ours up, so I can't remember exactly. Punch F1 - maybe the help will give you some clues?
I know I had to use the Revert N feature because our dispatch is notorious for punching the tones twice (which means you get two stored-voice recordings for each dispatch), then waiting quite a few seconds after the second tone (letting the repeater drop, of course) to actually read the call out (where they tend to dispatch in pyramid style - give you 47 details you don't need, then two minutes later finally tell you that it's a 737 crashed in a field somewhere
)
If that doesn't help, let me know and I'll do further research. You will need to "play" with the programming settings a bit to get them tweaked the way you like them. I used my service monitor to generate tones (1mW on the repeater output) to get ours the way I wanted them.
I looked at the "help" file. I currently have one set to "revert". I will try that. If not any good, i will try "revert N" and specify time as long as i can go.