A key concept behind agile practices is that once an iteration starts, you try not to change its scope. After all, it is only a short time until the next iteration. Why can't the interruption wait? By adhering to this approach, it allows the agile team to focus and not get killed with interrupts.
How does this approach work with third level support (which by definition works on an interrupt basis). You usually can't wait until the next iteration to start working the customer issue. To do so would typically violate your SLA's.