I've had this happen a few times, seems rather odd to me.  Pictures tell a thousand words so see below. These pictures were taken in my office, in Hillmorton/Hoon Hay, Christchurch.  Phone is a G1 with Cyanogenmod.  And to be clear, this is a Vodafone on-account device, not 2degrees.  

It seems to happen when I have the phone shut in a big cabinet with metal sides and top (plastic doors, USUALLY it gets a signal just fine in there, infact a pretty good signal) so maybe something to do with signal problems on Vodafone network and then it magically "roams" to NZ Communications.

Sorry the images are out of focus, had to taken them using my netbook's built in camera.  In the second image the signal bars have an R in the top to indicate roaming.  

Calling does not function in this state.

Maybe my "server" cabinet opens a worm hole to some far flung destination when I close the doors?