I don't think its paranoia. In my case, and it seems a few people are finding it similar, it's usually someone trying to sell you something you don't want - GAP insurance from the car, pension or policy from the Bank, mobile phone provider, etc.
When they dont leave a message, it is more than likely a sales call from a call centre. When I was in Australia in January, someone kept calling my phone at night down there(day here) and waking me up and not leaving a message. Obviously, no numbers came up at all, everything was coming in as call, so me thinking something was wrong at home started calling all the family. It hapened 3 nights on the trot. I answered on the third time because I was awake waiting at this stage only to find it SKY tv trying to sell me a package. Most annoying.