Transmitting original calling tel number to transfer phones

I need to pass the originating number ( external ) to the transferred phone. I used callerid(num) but this gives only the previous number , not the original nr
How can this be done

From the book it seems that putting an “o” in the dial application (3rd argument) will work, but I’m still experimenting with it myself.

Uses the Caller ID received on the inbound leg of the call for the Caller ID on the
outbound leg of the call. This is useful if you are accepting a call and then
forwarding it to another destination, but you wish to pass the Caller ID from the
inbound leg of the call instead of overwriting it with the local Caller ID settings.
This was the default behavior on Asterisk versions prior to 1.0.

I also already tried with the o flag but i cannot make it work
it seems to me such a basic and fundamental question that I assume there must be an easy way. Off course I am not a in depth specialist
In my case I want to send the phonenumber that is originating the call, together with the last one that is picking up the phone so that I can send a message to the correct user ( phone)