All calls on Samsung F480 appearing as "unknown" number

My phone wasn't displaying any numbers when I received a call - even though I had corresponding entries in the phonebook. For the benefit of others googling for a solution to this problem: it isn't your phone. It doesn't matter if your contacts are in the phone or in the sim. Most likely you need to call your provider and make sure caller id forwarding is enabled on your service. This can happen when you change providers or start a new service.