No caller ID

[Anonymous]

Hi,

I received two calls yesterday and today no caller ID. How can I know who called me, please?

Thanks for help.

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[jonsie]

The problem with blocking unknown numbers is that many multi nationals and NHS  services withold their number.  You can block them  all but may miss an important call. 

[MI5]

Have a look for an app that will block unknown numbers if that's what you want to do, but heed the advice from jonsie about the potential to miss some important calls.
O2 can't do anything about it unfortunately.