I've been on FaceTime a few different times when another phone call (not FaceTime call) comes through. I've never tried to answer it. Does the caller hear anything different on their end? Will my voicemail still answer the regualr phone call? Does it notify the caller I'm on FaceTime? Will texts and/or iMessages come through while on FaceTime?
iPhone 4, iOS 6.1.4
When you receive a phone call during a FaceTime you are prompted to End FaceTime and answer the call or Ignor the call.
If you Ignore the call the caller will be directed to voicemail. They will not know you are on FaceTime, and no messages will be sent unless you explicitly send one yourself.
If you choose to Answer the call FaceTime will end and the call will connect. The person on FaceTime will not know that your on a call now, again unless you explicity tell them. They may call back and you have the option to Ignore them. If you ignore them they get the "Mr. Smith i snot available for FaceTime" message.
If you need more help see:
What happens when I'm on FaceTime on my iphone 4s and a phone call comes through?
