Really sometimes these unwanted calls are very annoying. To block unwanted calls you can try below mentioned methods:
1. If you are facing this problem and want to block these calls from your phone you can block calls from your phone setting. For this Go to the call settings and click on the option "Caller ID and spam protection" in Android phone. Now your phone will identify all incoming calls and block spam Calles automatically.
2. The other method for blocking unwanted calls you can also activate DND service through your mobile network operator like Airtel or Idea etc. For this you will have to customer care to your mobile network operator and make a request for the same.
But sometimes these blocking methods may also block some important calls like from your relatives, friend or colleagues who have changed their numbers.
3. If you are getting calls regularly from few companies or individuals, you can also block those unwanted calls individually. To do so, open the phone app on your Android phone, long press on a number that keeps calling you, and tap on "Block".
4. There are several apps also available in the Google play store. Some of them are free also. A these apps are really very useful as they provide their name and you can identify the call be their names. You can download any of the following app.
a) Google Phone
c) Blacklist Plus
d) Call Control
g) Call Blocker
h) Mr. Number
i) Calls Blacklist
j) Should I Answer?
l) Avast mobile security
Download any of the app and click on the settings and select "blocking". Then you will find many options like Private numbers, Unknown numbers, all calls etc. You have to select on "Unknown numbers". After clicking whenever you will receive a call from a number not saved in your contacts, your phone will not ring, but you will be able to see the call.