When configuring the Quintum A800 stay away from the GUI, nothing useful can be done here. Everything should be done from the Command Line Interface (CLI). Although this initially seems quite daunting the commands are broken up into logical sections, one for the pstn interface, another for the dial plan etc.
The simplest way to start is to use somebody elses working configuration, a complete print out of the settings can be obtained by typing in the following at the root level CLI prompt:
> co [return]
> print [return]
I have a number of working configurations for the A800 that I can mail you – this will get you up and running quickly. Please send your contact details to me at: firstname.lastname@example.org , I will forward you some configs by return.
You mention that you are using a DSL link for the A800, remember that if you are using G.729 you would require a minimum of 150.1kbs for eight simultaneous calls (this presumes a 30% silence suppression). This can be taken down as low as 74.8kbs if you install the PacketSaver option on your A800, although it does require that PacketSaver is also installed on the gateway you are sending to or receiving from.
Hope this helps.