Asterisk ringgroups - weird problem

I’m running on asterisk now, very basic system. Two extensions, four incoming lines. All lines are set up to ring both extensions every time. If you call, both extensions should ring.

But that’s not what happens. Sometimes - only occassionally, only extension 2 will ring and extension 1 will not ring.

This seems to happen only when the incoming caller ID shows ‘anonymous caller’. I can’t imaging that there’s any correlation there, but it’s the only thing I’ve noticed that seems to be consistent. However, not all anonymous callers only ring one extension. Most of them work as expected, and ring both extensions.

How/why would I be getting the occassional inbound call where only one extension in a ring group actually rings?