The way it will probably work, in my experience, is that you will go a few days with no activity if it was picked up, then in the span of a few hours it will be constant calls all over the world, or roaming from someplace very far away.

Of course, it could just be lost.

I would have them deactivate the sim, and issue a new one. If you know the phone is missing, should somebody have it, you may be responsible for those calls, and it won't be cheap (and you are assuming they will deactivate it quickly enough).