Top 3 traits is
Have thick skin, you can never loose your temper or step outside the law ever not even once.
Learn from each call, what did you do good? what did you do not so good? Is there one thing that could have changed the outcome of that last call?
Keep score, know your numbers, Dials/talk offs/deals even if using auto dialer you can keep score it is so important.
Set goals and work towards them everyday. Learn Empathy not sympathy.