Dear Mr. Fancy Pants Big Shot Corporate Guy,
I am sorry that [insert name of really important engineer] is not available to take your call. And I am sorry that I cannot personally walk up to him while he is basking in the sun on his vacation in Seaside, FL to let him know that you would like to speak with him. Also, I am sorry that the six other really important engineers that you asked to speak with are not available to take your call. Rest assured, if I could I would travel to the office where they are sitting, undoubtedly having really important meetings with other really imporant engineers and architects, just to tell them that you really need to speak with them. Contrary to popular belief, I do not control these really important engineers, and cannot pull them out of my ass so that you can get in touch with them, or make them magically appear in their offices. Thank you for your time and understanding in this matter.