Truecaller - Caller identification service - System design - Part 1
Functional requirements:

1. Display caller id of the user
2. Caller id should be as accurate as possible
3. Search for name of a phone number
4. Identify spam caller

Constraints:

1. Total number of users: 3.5 billion
2. Total number of phone #: 10 billion
3. Total number of phone calls placed every second: 50 million