You've probably figured out your issue by now, but for others coming to this thread, it may be that Three have switched off 3G in your area and your phone can't support 4G calls except by using data. So you might not receive calls (no ringing, nothing in call log), get a voicemail notification message, but not be able to call voicemail to retrieve it, all while having apparently 5 bars of signal. This seems to confuse Three helpdesk, even though it is down to Three switching off 3G. If like me you're a very low user of your phone "as a phone" and you're with Three as one of the last providers of a genuine PAYG offer, it's also frustrating to be told that the only way round this is to buy a data bundle!
If you switch your network preferences to 2/3G rather than 2/3/4G you can see if there's still 3G coverage in your area. It looks as though I might have only been able to make and receive calls for the last few months when I've been abroad and/or out in the boondocks with no 4G signal
