Why do some incoming calls go to one phone instead of the ring group?

I have several phones including a Cisco 7940. I have Trixbox set up to send all incoming calls to a ring group that rings all phones. Normally, everything works fine but for some calls, only the Cisco rings. It seems to be related to the calling party's phone. For instance, when my daughter calls on her cell phone, only the Cisco rings. If she calls from another phone, all phones ring. Her CID shows up as 0000000000 but I also have this problem on some phones that show the caller id.
Does anyone know what might be causing this?