Almost since inception about 5 years ago, I forwarded ports 5004-65000 on my router to my ATA. I created a fixed IP on the ATA and that's the IP the router port forwarding points to.

I haven't had any port conflicts that I am aware of and have had perfect phone service ever since.
Wish I could take credit for this but the lead tech at VOIPo advised me...and he was correct....as he always is.