Returning that ex-girlfriend's phone call all depends on one factor: sanity. Your sanity -- as such, would it really scramble your brain to hear her voice again? Her sanity -- is there any left? Could its tattered remnants make you feel like shit again?
Take note of the overall tone of the message. Is it punctuated by the sound of cocaine being snorted, or by the sound of a weapon being loaded? These are not good signs, and do not qualify the call for a callback. If the tone of the message is bright and retains some of the sweetness you used to share with your ex-girlfriend before she became an ex, then the call should be returned, barring multiple personality disorder or if that sweet voice is saying "I'm in jail, could you come and bail me out?"
If the call is regarding the computer that you built for her, do not under any circumstances return the call. You'll get suckered into going over there and fixing whatever she did to break it. Unless you're a true masochist, just knowing that the time, effort, and precious hardware that you put into that computer can never be recouped is punishment enough.
Different circumstances call for different actions, of course. However, caution should always be extensively observed. You're dealing with someone who probably knows a secret or two about you, has some friends in common with you, has seen you naked under good lighting, etc. This can be a thin line to walk, and it should be walked with due speed and utmost discretion. Whether you call her back or not is up to you, but just remember -- it's probably not worth your time to do so. Instead, you should meditate and think deeply about how much you enjoy being single.