Hi,
Just a recent item I had to deal with here at home when faxing thru my DIR-655 using Vonage....
I was having real hit or miss issues when faxing... Have fax set to high compatibility, yet I was getting faxes rejected prolly 80+% of the time...maybe even higher failure rate.
Did some digging on vonage support pages, and they had an item stating that if your router's firewall had SIP "filtering" enabled, to disable it....and for me it worked. Now my faxes go thru with about zero issues...
SD1