Inbound and Outbound to one specific Extension

I have FreePBX based VoIP System have Astra card with 8 port FXS and having 8 POTS telephone lines. For senior management i want to use one number dedicatedly assigned for inbound and outbound. I could achieve the inbound but outbound iam not able to configure to use one specific number for that extension.

Extension : 111
Landline : 08023981999 needs to be routed inbound and to make outbound calls i want to use the same number from that extension and this number should not be used by any one else.

How to achieve that, please help!

Setup a trunk for just that DAHDI channel (just the channel number in field) then setup outbound route for the one phone (put phone ext in CID field of outbound route).