Pigro, Have you had any issues whilst connected on a 2G connection? Maybe you should put it to the test so that way we could see if it really is a fault with network or the device, that is if you have the time to do so :-)
tested this (Vodafone UK) this morning by starting a call on speakerphone from n900 to my landline, leaving it open, and then kicking off a remote control session (rDesktop) on n900 over a 2G connection (5 bars).
So, Voda UK is definitely able to cope with simultaneous voice/data on 2G, and the n900 is not to blame if your cellco doesn't allow this.