Most typically follow Medicare's clinical coverage policies, any many of the big payers have their policies available online. If you approach it as "I need to be able to tell the doctor/patient/whoever why this wasn't covered. Is there somewhere I can look to find your policy on this?", a lot of times they'll help.
If they're still uncooperative, then don't be afraid to get assertive. remind them that you are not calling for their opinion, so "I think" is not what you want to hear. You need an answer. "That's just the way it is" is not an answer. You are not 5 years old - "because I said so" isn't good enough. And you're capable of reading the EOB yourself, so you didn't call them to hear what you already know. You are legally entitled to full diclosure on all of their payment policies in writing. Tell them that (verbatem).
If someone tells you something that sounds wrong, it's probably wrong. Keep in mind that many payers outsource their customer service call centers (Cigna uses Convergy's). The person you're talking to has likely been employed for less than 6 months (if even 3), and a good portion of that time was spent learning how to navigate their computer system and procedures on call etiquitte. They sound knowledgable because they're trained to. If you aren't getting the answers you need, ask for a supervisor. Make them explain every detail that doesn't make sense until it does (playing dumb helps). That's what they get paid to do. Go all the way to provider relations if you have to. Sometimes that's the only way.
One more thing - don't be afraid to bluff on what you know when you're on the phone. As long as you sound confident, they'll take your word for it 99% of the time. If they call you out on it, who cares? Hang up the phone and call back to get someone else. They don't know you, and they'll forget about the whole call by the end of the day. As I mentioned before, their opinion doesn't matter.
