Think outside the box mate. When it comes to best achievements that is usually the way to go. For example I once sat in an interview and said that my greatest recent achievement was conquering my fear of flying and IBS to fly to Rome with Claire for her 30th. My point is don't get hung up thinking they will want something academic or even necessarily work related - just think of something in your life that you feel genuinely proud of yourself for having achieved - this question is more about a genuine and heartfelt answer than it is about the content, and if you sit there and come out with a pat answer about devising a new filing system at your last job that increased efficiency then they will spot a mile off that you're just making shit up that you think they want to hear.
As to why you want to work there - that's a bit more difficult. This is the part of the interview when you DO have to bullshit them because lets face it the honest answer for almost any job to that question is 'so that I can earn money'. What you have to do is say stuff like 'looking for a fresh challenge' 'enjoy helping people' etc etc.
At the end of the day, I have worked n a call centre environment, and its as dull as ditchwater - however, it can also be a really easy ride if you aproach it the right way. Often you will find if you have quiet periods you are allowed to just take out a book or whatever. Most of what you have to say to people will be scripted so you don't have to think too much, and they usually offer fairly decent benefits precisely because otherwise nobody in their right mind would ever stay.
So try and enjoy it - its just an interview after all, nobody will shoot you at dawn if you don't get it. Worst case scenario, its good interview practice for the next time, best case scenario you get it and it keeps you busy and earning while you wait to find something better
Good luck!