AT&T SB67138 - Manual (Page 73)

Caller ID
Reasons for missing caller ID information
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Reasons for missing caller ID information
There are also occasions when other information or no information shows for
various reasons:
On-screen message Reason
Private caller The caller prefers not to show the telephone number and name.
Unknown caller
Your telephone service provider cannot determine the caller’s
name and telephone number. Calls from other countries may also
generate this message.