First, register your mobile phone number with Do Not Call Registry at Personal Data Protection Commission Singapore. That will prevent unsolicited phone calls, SMS, and fax.
If you don't want to register to Do Not Call and would want to have more control, then the second option would be to utilize your reject list setting on your Android phone (I'm not familiar with other operating systems).
Here are the steps.
|Steps taken to set auto reject to Private Numbers (or Unknown).|
However, be warned that if you reject Unknown (or Private Numbers), you will not be able to receive calls made from Singapore public phone booths. Strangely, calls from public phones are under Private Numbers (or Unknown).
To add a specific number to reject list, it's easy - Just go to Call Log and touch on this number. A pop up will appear showing several options with one asking "Add to Reject list". Touch that option and the number will be added to the reject list.
|To select a number from Call Log to be added in the Reject List. Go to Logs and select the number by touching it. A popup will ask for several options. Select Add to reject list.|
The thing about Reject list is that even my three year old toddler can do it. My number was selected in my wife's phone and added to Reject list. When I wanted to call my wife, I would get the "Line Busy" notification. I never thought that my call was rejected until I tried numerous times. This had happened twice (palms on face).