What are personal callers in this context..what does it mean..please help?

Question

A limited number of tickets for each session are available on the day from the on-site box office, to personal callers only. The box office will be open from 09.30.

Answer 1

It means you will need to go to the Box Office in person and queue to get one of a limited number of tickets.  In other words you won't be able to book them online or over the phone - you have to turn up and buy them over the counter.

If the tickets are in high demand, then get there nice and early as there will be a big queue!