NEC Univerge DT750 User Manual page 166

Note:
When making a call from Call History, a prefix
can be added to the dial number. In that case,
select 3 Prefix on the SubMenu (indicated at
STEP 2 of CASE 2) to display the Prefix screen.
On the Prefix screen, choose a prefix to be
added to the dial number. Also, prefixes need to
be set in advance. For details on setting of prefix,
see TO ADD PREFIX TO NUMBER in Chapter 6
Directory Operation.
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