You are being called from an overseas call centre which is automatically dialling lots of numbers simultaneously. If, when you answer the phone, there aren't enough staff to speak to you, you will either here a dead line or the background noise of the call centre.
When you receive an incoming call the line remains connected until the other person terminates the call. If you put the receiver down, and then pick it up again, you'll still be connected to the incoming call.
Calls received from overseas (particularly from call centres) usually don't support the 'caller ID' function. If you dial 1471 with BT you'll get an announcement telling you the time of the call and that "We do not have the caller's number to return the call". However Virgin choose not to provide you with such rather useless information. Their system ignores any calls with no 'caller ID' and instead tells you the number of the last caller whose phone did provide them with a number. (In this case, your husband's mobile).
Chris