Here in Brazil, using the operator VIVO I have the same issue.
When using 2.5G or 2G, connected to the Internet with something simple as a ping or ssh session, all calls to my phone go to the voicemail.
I have to use my old BlackBerry 8310 when I travel to 2G and 2.5G areas, so I can use Internet and receive voice calls. I just use my N900 now when I am home (where I have wifi and 3G).
I hope we can see some kind of solution or work around soon.

Best regards!