I believe, but I may be very wrong on this count, that we can only be called on three times. I managed to get called three times in 4 years.
Once was during the exam fortnight at Uni.
Once was during semester, but it was estimated it would go for 5 weeks (!) so I got out of it by saying I could not miss that much Uni.
The third time I went in with about 50 people, and they pulled names from a hat, they asked both parties if any of the people brought forward were problematic to their case, swore the accepted people in, and sent the rest of us home. A week later a cheque arrived for $35 as my payment for showing up.