If the unknown number if from a large establishment, it may come up in a search. The first thing you should do with an unknown number is type it into a search engine and see if it yields any recognizable results. You may find that a local companies and large business, such as your bank, was trying to contact you. If you're on Facebook, you may actually be able to use it to use this to identify an unknown caller. Simply type the number into Facebook's search bar. You may find a profile linked to the number online. Keep in mind, this does not always work as some people's privacy settings prevent their numbers from being linked to their profile. If you type "reverse phone lookup" into a search engine, you should find plenty of sites that allow you to type a phone number in to identify the caller. Try a few of these sites to see if any of them yield helpful results.  Reliable sites include sites like White Pages, Reverse Phone Lookup, and AnyWho.  Some sites may not be able to give you the exact name of the caller, but can provide you with a general location of the caller. This can help narrow it down. For example, if you know a classmate of your lives in a particular neighborhood in the city, the number may be linked to that area. If you recently gave that person your number, they may be calling you.

How would you rephrase that briefly using Vietnamese?
Nhập số điện thoại vào công cụ tìm kiếm. Nhập số điện thoại vào Facebook. Sử dụng một trang web tra cứu điện thoại đảo ngược.