In general, your "sip:8xxxxx@voxalot.com" forwarding looks correct. On FreeDigits, they don't want the 'sip:' at the front. On IPKall, they want it as two fields (8xxxxx and voxalot.com) and no 'sip:' anywhere. IPKall can take an hour or more to recognize the change. FWD should be "sip:8xxxxx@voxalot.com", but FWD has been having a lot of problems for a long time now with changes on the 'Extra Features' page not taking effect (ever).
I don't understand the following statement:
"FWD only works when I call to my ATA at
8xxxxx@xxx.dyndns.org but forwarding fail to work even when I set to my ATA address."
Ron