Overflowing a ring group to itself

[Dogbreath]

MiVB 9.4
I have a ring group that has been [probabiy inadvertently] programmed with an overflow point of itself.
The call rerouting for it is 1/1/1/All/1, ie just ring here forever.
It has no call forwarding profile.
It has no active features according to locate feature extension.
There is a mailbox with a matching number.
The mailbox forwards to email and the destination address is unique, ie no other mailbox emails there.
From the logs on the mail server I can see that it is definitely MiVB sending these messages.
From the subjects of the emails, I can find the caller numbers in SMDR and see that they rang this group.

So given all the above, how does the system know to send the calls to voicemail?

[Dogbreath]

I had forgotten about the Call Flow feature.
The mailbox for this ring group does not itself have a call flow configured, but all calls to this group come via an IVR built with a call flow. The call flow just transfers to the ring group, there is no other logic here, but I guess that means the call is already being handled by the voicemail system, hence the call landing in voicemail.