Prof asked to interview me, now I can't get in touch with him - what's the etiquette? So here's my situation. I'm currently finishing up my masters (and that's a tale for another day) and have applied to a few universities for PhD.

Late Feb I was emailed by a postdoc who wanted to interview me on behalf of his prof who was inundated with grant applications and whatnot. We spoke on a week later, that went well, and I was told that if I was successful, I would hear from the prof himself. The following week (on a Friday) the prof himself responded to me that he wanted to have a follow-up conversation with me about it.

I replied that Sunday  letting him know I was free any time. I heard nothing, so I sat on my hands for a week and sent another reply letting him know that once again I was available that week if he wanted to talk. He informed me he would let me know but was on his way to a conference. This time I gave two weeks, and yesterday (April 7) called twice, about an hour apart, leaving a voicemail the second time, and sent a follow up email about an hour later as well. So, it's now been almost four or five weeks.

His voicemail mentioned an assistant that I have not contacted and I have likewise not emailed the postdoc since our previous conversation.

Is there anything left that I should do about this? How should I take this? 

When I was interviewed by profs the first time I applied for grad school I don't recall having these issues. I want to be assertive/not look disinterested but also don't want to overstep any bounds. How responsive should I expect a potential prof to be?

To my knowledge this question has not been investigated elsewhere on this site.
I had similar issues with contacting my future PhD supervisor when applying and found the most efficient route was contacting the assistant. The professor is clearly busy but definitely doesn't want to be inundated with your emails. The assistant may plan his schedule and can book you in. Contacting the post doc isn't as efficient and can look like you are sidestepping the professor.