I've called exactly at opening many times (8:00 am currently), and called back over and over in the morning, even still rarely get in the call queue. Not sure how many people can be waiting but it's almost always instantly full.

That is the only way I've ever gotten in the queue so far though, calling multiple times right at opening hour. And then once in the queue I see the issue above re: no answer / mailbox is full after waiting on hold for ages for my turn.

I also have a thing on my phone that automatically pushes the right buttons as fast as possible to navigate to the extension I need (for a more efficient rejection).