Incoming Call Intermittent Failure on VPN

I’ve done some more digging, and I’ve found that I’m having a similar problem to what was mentioned in this post. Except that I seem to be having an intermittent problem. Can anyone offer advice on what exactly @hawkeye was describing to do? Is it a server side change or a phone change? I don’t have DTLS enabled on the server, but neither do I have TLS enabled.