I had not used my phone service in a while (many months). When I went to use it again, I found that I was unable to place outbound calls, or to receive inbound calls. Upon lifting the phone from the hook, I would hear a dialtone just fine (and the "voice" light on my ONT would glow amber as expected). But as soon as I pressed any number (0-9), it would immediately begin giving a busy signal.
In the end, after contacting Sonic support this evening, we were able to "fix" the issue by simply power cycling the ONT. As far as I know, the ONT hadn't been power cycled for many months.
I'm not sure how my ONT (an Adtran 411) got into this state. (By the way, the Ethernet connection was working just fine.) I'm hoping it's an isolated incident. I'm posting this report (a) in case anyone else runs across this problem and is looking for a quick fix, and (b) in case others have experienced something like this.