The problem you describe is one of the problems I have been experiencing. I believe that I often have calls that I NEVER hear ring. those that do ring, go to voicemail too quickly for me to get to them.
I can't help beyond what's already been posted in here and around the forums, but I just feel like posting some things that cross my mind about these issues.
First and foremost with regard to the Eris itself (something I keep repeating because it seems to help explain some of the ongoing issues with this particular device): great design, good looking, but assembled and inspected poorly, thus the spotty reports of its reliability. Quality control, for some reason, failed many users at the point of manufacture and assembly.
Also, the slot cycle index setting determines how often the device polls the nearest tower for incoming calls; it has nothing to do with signal strength or quality.
I think some people with missed calls, calls going to voice mail without igniting a ringer and dropped calls are suffering poor signal. That can be due to so many reasons that it is mind boggling; reasons in the phone (back to poor quality control by HTC) and reasons not in the phone (weather/atmospheric conditions, proximity to tower, tower quality, etc).
We've tested this at work and at home with Verizon and Sprint and with several devices over the years; it's remarkable how I can be on a call in the below-ground floor in a huge hospital with two bars signal strength on my Eris, and right next to me is a lab tech with a "dead" Incredible or a doctor with his Blackberry of no use to him: no incoming or outgoing calls possible on their devices.
I've even called and summoned a friend from his house with his Eris, on the 2.1 OTA, to come to my location, and have seen his Eris die right there as I text, make calls and browse the internet with no problems on my Eris.
Updates probably can't fix it, so the crap-shoot of exchanging it for another one, one which was quite likely returned defective and could itself remain defective, begins; thus the posted stories in here of multiple visits to Verizon for yet another "refurb."
I would not stick with the Eris if I exchanged it due to serious issues and found the new one to also have issues. I'd kindly explain that I need another model of phone that has not suffered so many bugs since its inception.
Those of us with good Erises keep them, and mine has honestly had zero bugs, no signal problems (the bars look low most of the time, but the device behaves as if signal strength is maximum) and no "silent call bug" or any of the other complaints we see reported.