This has sounds like a problem that’s been reported before. Seems to me there was a lot of discussion on this a couple of months ago. I don’t remember the resolution, or even if there was one.
The logs will tell you what is actually happening, so maybe you could show us a call where the caller doesn’t hear a ring tone? Also, since this sounds like a bug, if you don’t get some kind of resolution on it in the next couple of days, maybe you should submit a ticket. In fact, if you check on the issues.freepbx.org website (where tickets are submitted), you might find the ticket where this was worked on last fall.
It doesn’t matter. Check the ticket database through the Issues tab to see if something has been reported here. The reason I’m suggesting that is because I’ve heard about this before, so it’s been reported here (I don’t follow the Asterisk forums). Check the FreePBX issues - if it’s there, see what they said. If it isn’t, submit a ticket, especially since it was working and now it isn’t. Even if it is an Asterisk bug, the FreePBX developers work closely with them and can coordinate a solution for you.