We never call.  It's as simple as that.  We will always email with a very specific question for clarity, and indicate that we are not proceeding with research until we have clarity.

That clarity must come in writing and be unambiguous.  If they cannot answer a question directly, then they are suspect as to whether they understand the needs of their own client, much less the industry's minimal standards and practices in such  matters.  That makes me wonder if I will get paid;  if they are unwilling or unable to put something in writing.

I always attach the full email chain to my invoices for the written verification of authorized work, so there is never any doubt about the work requested, when and how.