*69 will give you the number of the person that last called - usually.
"Unknown" on a caller display phone typically indicates that the caller's identity is not being transmitted to your device. This can happen for various reasons, such as the caller choosing to block their number, an issue with the phone network, or the call originating from an unlisted or private number. It could also mean that the caller is using a service that does not reveal their number.
I am not sure, if the number displayed is a valid call ID number, but Caller ID displays the phone number of the person who made the call to you including the area code and phone number. From this you an identify the person calling you. If you have a phone book system in the phone and you have saved phone numbers with names, some phones will display name of the person from the address book when the caller id matches the stored number. Some times the number displayed on caller ID is not a standard phone number. Some of these calls may be from a VOIP service or from some customer service call centers (internal on the service provider network) or when the caller ID is masked by the caller or caller's network service provider. - Neeraj Sharma
Star 67 is a feature on some phone systems that allows the caller to block the display of their phone number when making a call. By dialing *67 before dialing the recipient's phone number, the recipient will see "Private Number" or "Unknown Caller" on their caller ID.
It defines whether your phone number will be displayed on someone's caller ID unit when you call them. Select 'no' if you wish your number to remain private.
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You can report nuisance phone calls in two different ways. You can report them to the police, especially if they are dangerous in any way. You can also call the phone company and report them.
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It is possible to "spoof" caller id. If you operate your own PBX or you use one of the IP (internet) phone services with lax security, you can change the number that shows up on caller id to be anything you want it to be.
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Basically you can't - the whole point of a caller being able to with-hold their number - is to prevent it being stored by the called party's equipment.
"star" 67 then the phone number you are dialing
Caller ID