(iax.conf)
[iaxfwd]
type=user
context=fromiaxfwd
auth=rsa
inkeys=freeworlddialup

(extensions.conf)
[fromiaxfwd]
exten => ${FWDNUMBER},1,Dial(your_extension)

or if the extension is in another context:
exten => ${FWDNUMBER},1,Goto(extension,s,1)