I need to know how to trace private phone calls on my phone.

If they are blocked or non published numbers you can not track them.

If you report them to the police or phone company they can trace them

If you report it to your cell phone company, they will not trace anything for you. There are laws about confidentiality.

The police themselves cannot either. What you need to do is have a lawful reason to track a number, take it to court and have them deal with it. They can request phone records for various reasons.

How Can I Find Out The Number For The Person Who Rang Me On Private Call

You can ask your service provider for automatic number identification. Its normally provided to business lines. Such as 1800 numbers, for credit card companies especially, because they want to verify that you're calling from your house.

Here is a post about exactly this topic:
http://www.cellphonelookup.com/blog/2010/06/25/how-to-trace-mobile-calls-without-the-number/

I don't try it myself but seems like it should work.

How can i find out who call my cell phone from an unblocked call? How can i get the number to a unblocked callers?

If they are blocked or non published numbers you can not track them.

If you report them to the police or phone company they can trace them

What happens when you know it is your sister in law and she says its not her doing it?

We know it is sister in law but she says she is not doing it?

What happens when you know it is your sister in law and she says its not her doing it?

Then we ask how you know it's her, since you don't have proof and she's denying it.

See how that works?

You can't know something unless you have proof.

You assume it's her.

Assuming is not good enough.