From the Multiline user guide:


Last CO/PBX Number Redial
1. Without lifting the handset, press the Redial key.
The last dialed number is displayed.
2. To redial the last number, press #.
OR
Search for the desired number from the Redial List by pressing the Redial Softkey or VOLUME UP or VOLUME DOWN keys.
3. Lift the handset or press Speaker to place the call.


DJ