Normally a caller's telephone number is passed on in a call and presented to the called party (if he or she has a CLI display on the handset). Under the CLI Code of Practice, a caller is able to prevent his or her number from being presented to the called party and in so doing remain anonymous.
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Please note: the PSTN Caller Display service only operates on hard-wired extensions not connected to the router/adapter (if you have DECT extensions it will not work on any of these); your Broadband Voice phone(s) will only show the calling number if you have set up caller display on Broadband Voice and the call is to your 05 number.