Life South of the Equator could be dificult indeed.
About the ATA and MTA 3328-2.
The ATA 186 have two diferent modes of operation.
One is using the protocol H323, that is normally a pain to setup when you have a NAT router(one public I.P. address from the NET, several devices or computer plugged into the router).
The other posible setup is called SIP, the most common one among the “new ” internet providers, like Vonage or those.
The diference betwen the ATA and thew other devices could be that they are running diferent protocols.
But my WAG (Wild …. Guess):If you get broken audio. It should not be blocked ports. Blocked ports normally do not work at all .
It is indeef extrange that you have trouble with audio one way, to the NET. Normally is the other way around.
Sugestions: Put the gateway device on a static I.P. address on the NAT router.
Make that I.P. address the DMZ zone .
That should make the gateway have a better connection to the NET.