Re: Same problem as any helpdesk.
"I can't understand why people keep paying for these fecking things though, you'd think someone would have a clue by now."
They keep paying for these things because they're easy to measure - the call centre equipment will do the measuring for you. Even excluding test calls, assuming the equipment can differentiate, won't help because the consequence would be hanging up live calls to take new ones.
Time to answer is probably a good measure if used sensibly. If you get towards the failure limit it tells you you're getting to the point where you need to add resources but, of course, this is going to be resisted by whatever management entity is going to have to pay. Turning into a target makes it useless as a measure (Goodhart's law rather than the Cobra effect).
Setting targets based on outcomes, which is what should be done, creates a very much more difficult measuring task.