Why is my ex-boyfriend ringing my phone then hanging up on my voicemail?

I dated this guy in 1998. We were on and off until I got fed up and married another guy in 2003. I ended up divorced in 2006. After my divorce, I was back on the scene so to speak - I went clubbing again and ran into my ex. My ex was friendly, but then had a public meltdown when he saw I was on a date. I got the feeling that he still had strong feelings for me. He told me that it was difficult to be just friends. He left with his friends and I never saw him again. Now, it is the summer of 2010. I pust my phone on silent during the night. I have seen his number pop up a couple of times at around 4 and 5 am. I texted him telling him that he must be pocket dialing me because we haven't seen each other in 4 years and that he should avoid sitting on his phone. He never responded. But his number showed up again. Is my ex-boyfriend metally ill? Why is he ringing my phone? I am very curious.....I am not very good at psychological issues and could use some input.