Steve in Herts formulated the question :

>
> Hi,
>
> I'm not using a router, My Grandstream ATA is connected directly to
> my cable modem,
>
> What is "DMZ" out of interest?
>
> Cheers
>
> Steve

Your ATA has a router built in <http://www.grandstream.com/y-ht486.htm>

DMZ is "De-militarized Zone". In your case, no use looking at DMZ as
your ATA is already in the best position in your network.

Have you tried any alternative service providers? If you set yourself
up with a well known free provider, you could establish (for the most
part) where your problem may lie - ring a freephone number to check
outbound & call yourself on the geo number to test inbound.