I have a similar problem but the calls get through but sometimes they can be appalling. I just turn off my connection to a Wifi network so the phone uses gprs instead (though a more efficient answer would be much appreciated) not sure if it helps but there is another thread with a very similar question with the solution...
Originally Posted by dansm77
It seems that UMA is a bit flakey, along with Orange customer service. Anyway, you can turn off UMA by setting the device to always connect to the mobile network, rather than wifi, i.e. Options -> Mobile Network -> Connection Preference, then select Mobile Network Only