Adding a number to beginineg of callerid

I am trying to add a number to the begining of a caller id in the extensions.conf
I have calls coming in on a 800 number and I am forwarding them to a mobile. This is working by making an IAX2 call out.
I am trying to edit the caller id to add the prefix 0800 so when the call comes through to the mobiel I know its coming from the 800 number.
What arguments do i need to pass to the SetCallerID string in the dial plan to add 0800 to the existing callerid. In other words I want to pass the incoming callerid with 0800 attached to it to the mobile.


exten => 5,n,Set(CALLERID(number)=5-${CALLERIDNUM})

Replace the 5- by waht you want before the callerid. Tested and working on Asterisk

Thanks. I knew there was a way to do it I just could not quite seem to get the syntax of the command.