Possible sources of private numbers:
1. Telcos - Singtel, M1, Starhub
2. HSBC Bank
4. Some property agents
5. Overseas calls
I think it is really silly to have private number. The other day I had 3 missed calls from private number. Subsequently the person called again. This time I picked up the call and the person said he was from Singtel. I said wow about time you guys called I have been expecting your call for 1 week. He said sir, we tried calling you today but no reply. I said I wouldn't know cos your number shows as private. As far as I am concern, this is the first time you are calling me!
So moral of the story private numbers are a complete waste of time!!!