I've noticed that despite all the claims in this tread that using voip for sending and receiving a fax is not dependable, that the user manual for my ATA, a Grandstream HT702 ATA, says that the fxs port can be connected to analog phones and fax machines, and that "The HT70X is a full feature voice and fax-over IP device." Also it "supports T.38 Fax." Also "T.38 compliant Group 3 Fax Relay up to 14.4kpbs and auto-switch to G.711 for Fax Pass-through, Fax Datapump V.17, V.19, V.27ter, V.29 for T.38 fax relay"

You can get the user manual from grandstream's web site.

Also, on pg 21
FAX SUPPORT

HT70X supports FAX in two modes: 1) T.38 (Fax over IP) and 2) fax pass through. T.38 is the preferred
method because it is more reliable and works well in most network conditions. If the service provider
supports T.38, please use this method by selecting Fax mode to be T.38 (default). If the service provider
does not support T.38, pass-through mode can be used.
Also, there are some settings that can be made, however I don't see any in the "basic settings" screen, which is the only one end users have access to. There are 2 levels of access, end user access, which voipo customers have, and administrators access, which has been changed by voipo so as to keep customers out.

I'm not sure what T.38 means, and whether or not voipo supports T.38.