I used to be in that role until last Sunday.
Trying to remember what they asked in the phone interview, I know it only took about 10mins at most. I think they were mainly checking that I had a good telephone manner so make sure you're very polite and don't use any slang.
I think they asked a question on working as part of a team and how well you worked in one. Also a time when you went out of your way to help someone with something.
There were then some questions based on basic maths and how you'd deal with a customer's query. They were mostly common sense and the maths questions they said it was fine to write down and take your time. Accuracy being more important than speed.
There was also one question where you had to listen to an example then recall some of the information that was stated, but again it was pretty simple.
Good luck with it

I'm glad to be rid of the place but it seems an alright place for weekend/student work.