I am using a PAP2T behind this one. Using 2.01 firmware(beta 30 Jun I think, cant check since I am at work and it has hanged, again...). Sometimes after incoming/outcoing call it will fail to connect to SIP provider "[1]RegFail. Retry in 30" until i reboot the DIR-825.
I am not using port forwarding since that did not work but uses the SIP-ALG option and it seems to work most of the time until an actual call is received or dialled.