Sharp UX-B20 - Manual (Page 67)

Caller ID (Requires Subscription to Service)
65
5. Special Functions
Blocking voice calls
If you have turned on Caller ID, you can use the Anti Junk Fax function
described in
Blocking Reception of Unwanted Faxes in this chapter to block
voice calls as well as faxes from your specified Anti Junk Number.
In this case, when a voice call or a fax transmission comes in from the number
you have specified as a "Junk Number", your fax will break the connection as
soon as it receives the calling phone number from the caller ID service (before
the second ring).
To use this function, enter the number that you wish to block as explained in
Blocking Reception of Unwanted Faxes (see page 68). Only one number can
be blocked.
Note: This function cannot be used if you are using the Distinctive Ring function.