It didn't ring on my first try, but later I called my ISP and then TP Link again and she sent me a link on how to forward ports 5060 and 5070.
After I did that, I reset my router and tried calling my MJ line again, and it rang!!
They told me the reason I wasn't receiving calls sometimes was the firewall on the router prevented my MJ phone from ringing, and that's why I had to open 2 ports for it. It seems this time it should stay working every day.
It took me about a minute to forward the 2 ports.
I had no idea if my ISP would have been able to help me with this but they did and it doesn't look like I'm going to need a new router.