Client is not paying

I recently started a consulting business, and so far everything has been fine, I have a few clients and I've been fine-tuning my process.

One of my very first clients, for which I have provided extensive services has suddenly become very difficult to reach once the invoice was submitted and overdue. His invoice (net 30) became overdue on November 19.

I reached out by email twice (after 11/19, mind you , he got the invoice 30 days prior and had plenty of time to discuss beforehand), and he finally responded with "Wasn't there supposed to be a 20% discount" to which I have responded positively, explaining it was already applied, and explained further and proved my point gently. It's not a complex pricing structure, it's very straightforward.

After my response, nothing from him for a few days, so I send a followup, and he writes "hi call me". I tried calling him - busy, says "I'm in a meeting, call in half an hour", so I call back after 40 minutes - declines call.

I wait a day, and try calling again - declines call.

I sent a followup email on Wednesday... Nothing up until today.