Depending where you live (globally) there could be certain laws that stop you finding out who called if the number has been withheld.

In the UK you are not legally obliged to answer a withheld number. I won a court case with my bank over that one. And if the same number calls you 3 times within a week (I think), it's classed as a nuisance call and is subject to investigation.

Best bet is to contact your network provider at first and see if they can monitor your line for a while. They can tell who the caller is, despite what some say about withheld numbers, by checking the CLI.

The Appearance of Unknown Number is from the service provider. If the number is not registered with them, then u will get it as : UnknownNumber
tnx