Unknown when I receive incoming calls on Iphone 5S
If you see "Unknown" this means that the caller is not transmitting his ID (i.e. number) to your carrier and hence your iPhone.

Not much can be changed unless you pick up the phone and ask the caller to change his Caller ID settings on his phone. 

In addition, some business numbers and calls over VoIP tend to appear as "Unknown". It is not the fault of your iPhone but from where exactly the call originates.