Comment 36 for bug 1612367

This part from the description of bug #1642635 sounds interesting:
"It seems to me that this is most likely to happen if I haven't used the phone in a while. If I answer the call (because I notice the screen) or make a call, testing immediately after the phone does seem to consistently ring."

I experienced the same. Once I couldn't pick up the call because of no ringtone. Then I called the same person after ~5 minutes and asked them to call me back in an attempt to reproduce the issue. They called after ~5 minutes, but that time the ringer worked fine. Also as I mentioned in comment #18, I got two calls within ~1 minute. First one didn't ring, but the second one rang.