... no, that's probably one of the worst ways to measure work burnout. A lot of people don't realize that's actually what's going on, more would not admit it, and still more would not admit it over the phone. So on and so forth.
Surveys/interviews can definitely be part of the means of assessment, but as things go it's often a pretty terrible way to track stuff like this, especially if you're just asking them if they feel burnt out. Humans are often quite bad at self-identifying problems and worse at reporting or acknowledging them to other people. Would be even more so in fields where work burnout is likely, since the people in them are under significant pressure and generally have a great deal of incentive (not the least of which is continuing to function at-least-minimally while burnt out, which can be the difference between, y'know, being able to eat or not) to not admit the issue exists to anyone, even -- especially, often enough -- themselves.
E: And the fun thing is, just as under reporting is a significant problem, so is over reporting by people primed by the question to respond in ways that give false positives. So your calling and asking is very much likely to throw out all sorts of bad data. You really want to rely primarily on statistics and behavior tracking for something like this, not direct interviewing. The latter can get really bloody unreliable on certain subjects.