One phone rings, one doesn't?

I have a 3 outlet phone circuit. The master socket is fine, everything works correctly in there. On the other two outlets, the phone doesn't (audibly) ring, but I can answer the phone and ring out as normal - just no ringing sound on incoming calls. It used to work fine, then one day phones in these two sockets just stopped (audibly) ringing. I can hear the master phone ringing, so I just pick-up and use as normal - any ideas?