Voicemail Call Duration and Credit Usage Query

I had a call that answered and then hung up it went to voicemail. The call length came back as 72000ms, which is over a minute and unnecessarily using credits. Is there not a way to determine if it is voicemail (from the message) and end the call? This also makes me curious about the leave voicemail feature -- if it can't tell it is in voicemail, how does that work. Additionally -- how do credits work.. is it rounded to the closest second and calculated as I understand it is 10 credits per minute.