How to trace the private number of someone who called you up on the phone?

Because of my business, I have clients who called me up on the phone but sometimes the number doesn’t appear.What appears on my mobile phone is PRIVATE.How will i know or trace his nmuber so that I will know his country of origin?

You can try to contact the phone service provider and ask if they can initiate a trace on the last number called and they can research it for you.