Exactly the same thing happened to me a year or so ago, except that the caller (a woman) wasn't silent, she was horrendously abusive. I stood it for a couple of weeks, but after the tenth call in one day I snapped, and called my phone company. I was out through to a special department, and the person I sopke to couldn't have been more helpful - she said the best course in the long run was to allocate me a new number. She did it immediately, and I didn't have another abusive call. I texted or emailed everyone about the new number and that was that.
Speak to Orange. The fact that you're on Pay as you go shouldn't make any difference - you're a network customer, and a nuisance caller is a nuisance caller.