My father just got a new handset (Nokia Lumia) with Virgin Mobile SIM card. He's having trouble calling his wife, who is on O2 with an iPhone 4. Every time she answers, the call instantly drops. She can call him alright, but can't receive a call from him. Other numbers (including landlines and other O2 numbers) work perfectly from the new handset, and the wife can receive other calls on her iPhone 4 without difficulty.