Can I receive calls only from a set of approved phone numbers?
One way you could do this is to use the iPhone's Do Not Disturb feature.

More specifically:


Launch Settings
Tap on Do Not Disturb
Enable the Manual option at the top
Tap on the Allow Calls From field 
Select the group1of people you want to receive calls from
Go back to the previous screen
Tap on the Always option near the bottom of the screen
Exit Settings 


1 You may need to create a particular group for the people you want to allow calls from. To do this you will need to use your computer or a 3rd party app. Another option would be to ensure you don't have anyone in your contacts who you don't want to receive calls from. In this case you would then select All Contacts as the Group at Step 5.