I have my Nettalk account(s) try the primary number (device) followed by a second try (forward) to ring a VoIP DID(set to no). Still works fine after two years. Don't forget that it takes a bit of time trying the first destination before it attempts the secondary.
If you set the VoIP to 'yes' then you must enter the SIP address for that destination. I prefer to just forward as if it is a POTS DID. From memory, I found it worked more reliably, way back when.