Thanks - my 640XL is Dual SIM as well. Massively useful when travelling. Put a low cost local SIM in there and designate that SIM as the one to handle the data. Brilliant. And yes you can receive and make calls on either SIM. And in the UK when I was on Three, a second PAYG O2 SIM or something was useful when Three had no signal, which was all too often ! A pity most UK retail phones are not Dual SIM models even thought they are available in other markets. And of course Apple don't do them at all.