Voicemail is somewhat hard to detect because the telecom system/carrier doesn't tell us that a call has been answered by voicemail. It simply appears as if the call is answered by the other end. Our system attempts to detect voicemail (when that feature is turned on) by detecting the "beep" that typically precedes the chance for the caller to leave a recording. (e.g.: "The person you have reached is unavailable. You can leave a message. Beeeep.") This obviously doesn't work if:
The carrier voicemail prompt doesn't have a "beep" at the end or has some form of other prompt to ask if the caller wants to leave a message.