If you receive a call from an unknown number and there is no caller ID associated with the call, there are a few ways you can try to figure out who called.
One way is to do a reverse phone lookup. This can be done for free on websites like Whitepages or TrueCaller. All you need is the phone number, and these websites will provide you with information about the owner of the number if it is listed in their databases.
Another way to try to find out who called is by doing a Google search of the phone number. This may provide you with some clues as to who the caller might be, especially if it's a business phone number. You can also try searching for reviews of the phone number online; this could give you some insight into whether or not the call is legitimate.
If all else fails, you could always just call the number back and see who answers. This isn't always foolproof (the person could have blocked their caller ID), but it's worth a shot!