Ring Group only calls first extension

I set up a ring group that has about 7 extensions, all SIP. I have the ring strategy set to firstavailable, and an announcement set if no answer. I would assume firstavailable means ring the first extension that is registered and not on a call.

When the inbound call is made the first extension in the list receives the call. That’s fine, but when the first extension is not registered the call goes to voicemail, even if another extension in the list is registered.

If I change the ring strategy to hunt then it appears to work properly. I tried both first available and firstnotonphone and both fail the same way.

Am I not understanding how firstavailable and firstnotonphone should function?