I have found that on Orange if you have the Internet open it doesn't ring. For some reason the whole of your bandwidth must be allocated to the data hence no incoming calls. This is true for 2G and 3G, also if you have it open in the back ground. So far a Simple solution is to download Dolphin Browser HD web browser (Which BTW is a free app so Im not trying to get anyone to buy anything). A colleague suggested it as a great browser and when installing it I was prompted to set it as default. After this I repeated all of my tests using this to view the internet and all of my calls came through.
I assume its been done this way to reduce download times and speed up the browsing experience but to be honest I haven't noticed any detrimental issues using Dolphin Browser HD. This is just my view but seems to work for me and again I have no affiliation with the Dolphin app or whoever created it, I just happen to have found that my phone rings while on the net when using this app. If you like try some of the others and see if you get the same results.
I did get a call from Orange after emailing the support address with Jonathan response and after running through some details and going through further tests with Jeanette I still had the same problem. They also confirmed that my number transfer went through ok. My Desire is the Black on Firmware 2.1 update1, Browser version Webkit 3.1 and Internet app version 7.
Hope this helps,
**My tests were simple, open the internet app and goto google.com while this was loading call the phone from my work land line. Missed call. Once loaded into Google.com call again without data loading i.e. noindicators of upload or download. Missed call. Repeated the above with Wifi on at home with same results. Also with Sync on and off same result. The background test I opened Internet app, input google.com and then while loading hit the home key. Again Missed call.
Repeated these again with Dolphin Browser HD installed and now working fine, all calls come through.