I use a macro to find out if a call is local or has to be send to my SIP provider.
If Local:
In the macro I check the asterisk database if the user has set a call forward.
If the user has set the call forward I open the macro again with parameters of the number stored for the callforwarding.
All works great except the CDR records they are not changed to the new call (to the call forward number). The billing time is counted on the incomming call.