A fellow Desire owner on another forum rec'd this response to an email he sent to HTC
I questioned HTC about this and got this reply today:
"Thank you for contacting us. In regards to the 2G connection, We can confirm that 2G connection is the same all over the world and there are no phones that allow you to connect to the internet and receive calls or sms in the same time, unless the network force the internet to stop and let the calls come through. This is not a limitation of the phone but a limitation of the network that doesnt let the calls come through. This is a phone that if used in a 3G area will allow you to receive and make call while browsing the net. If used in a 2G area,like your case,will be up to the network wether you can or not connect to the internet and still getting calls or sms. I trust that this resolves your query, please do not hesitate to contact us again if required. Thank you for contacting HTC."
Basically, as we thought, if you happen to have an active data connection working on 2G when someone tries to call, the call will go to voicemail. It *is* possible for T-Mobile to implement the system whereby the data connection is cut off when you receive a call, but they haven't and I think the chances of us getting them to do it is somewhere between nil and zero.
T-mobile are going to have a problem on their hands as more and more people pick up Smartphones which behave in the same way as the Desire.