[Effendi]

With UM you start paying only when the person you are calling answers the phone (or if an answering machine answers, of course). You don't pay for free/busy tone.
So if your husband just wants to ring you, without you answering the phone, he'll spend nothing. But I don't get the point in buying a UM simcard for doing this... he can do it also with a Serbian card which would cost him less.

I suggested to buy 2 UM cards to call each other at a good rate. But buying 1 doesn't make much sense. He can get a Serbian card and call you on your Bouygues card at cheaper rates than with UM.