I also want to know how to send extension number as e164 calling ID as well. V3 - the default setting is done. However, this setting disappear after v3.1.
There is not found any valuable contains the extension number at "Gateway/Provider Outbound Parameters".
The workaround is put your outbound id as extension number. Then set up :
Gateway / Provider Outbound Parameters
From : User Part => Value select CallerName instead of CallerNum
IMPORTANT: According to 3CX support, this is a wrong setting. But we have no other choice. Set it as your OWN risk.