We all know it's good business to be responsive.
You didn't say if this is an agent you already have a relationship with....or someone you found on the internet, or someone who was referred to you.
Since you are using his name in a public forum, I wonder if you really want to reach him or simply embarrass him.
If you really care to hear back from him, and he has been unresponsive, say , after 3 calls, then ,try calling his manager..........if you are involved in an active sale or transaction with him..........for sure call his manager or broker right away.
Otherwise.........unless he has an emergency which he will eventually explain to you.....move on to someone new.
Even if an Agent is super busy, he should have an auto-responder to let you know when you can expect a return call. To not respond in a timely manner is unprofessional.
If someone calls me at the office and leaves a message , I check messages a few times a day. If they call on my cell phone, I notice a missed call and call as soon as possible , though not when I am with clients., unless it is an emergency.