This is one of the best usefull app for all the android users. I have been using this since years and it's very helpful. It shows the details of the unknown caller like his/her name and the place from where the number belongs, and it's accurate 85% of the time.
This app also helps me to block various calls from the company and some irritating people. Not only calls, but sms can also be blocked. You can download the app for free from the playstore and this app also gives premium features if you pay a little for it.
It shows caller ID if you are connected to the internet, directly when the bell rings and also after some missed calls.