Not much you can do here but this is a good area to emphasize the point that subtle changes in how the question is asked can bring totally different results.
A lot of average or poor recruiters have a vague idea of the move to make here but it comes off something like this:
“When will be a good time to call back?” This is a poorly stated question that puts the applicant in charge. He/she can reply in many ways; “Not this year, I'm on a hot project.” Or, he/she may just say “next week” and continue to waste your time.
“What time do you get to work?” or “Could I call you tomorrow, first thing or would tomorrow afternoon be more convenient?” These are much-better questions because they assume a time and only give the opportunity to respond positively.