I have never been able to get fax to work over a voip service that goes through the internet like Vonage. I have gotten fax over voip to work when going over a local network. To get those to work we have had to set the fax to a max transmit and receive speed of 9600 bps which was the max the ata would support. If we left them set a defaults where it would start trying at 33600 bps they would always fail.