Start open duplex conference feature
From Zenitel Wiki
| Feature: | Duplex Conference |
| Default directory number: | 8300 - 8320 |
| Feature number: | 88 |
| Parameter 1: | 1-20 = Conference number. 0 = Conference Off |
| Description: | Join a Duplex Conference by dialling the channel number (default 8301 - 8320). |