Steve in Herts brought next idea :
> Has anyone had any success using the voip.co.uk service in conjunction
> with a NTL residential Internet account?
>
> I am using a Grandstream HandyTone-486 ATA which connects ok to the
> voip.co.uk SIP server. I am able to receive calls from PSTN lines but
> when I try to call a telephone number, I hear the line ringing and
> then when the person I am calling picks up their phone, the line
> immediately drops. It has been like this ever since I signed up last
> weekend.
>
> I have heard only positive things so far about voip.co.uk on this and
> other forums so am beginning to wonder if it is anything to do with my
> NTL Internet connection. Does anyone know if NTL are blocking any
> ports which could be causing my problem. Or could anyone suggest any
> other possible causes?
>
> Many thanks,
>
> Steve

I use tem on Telewest with no issues, one thing I did have to do was
place my ATA in the DMZ on my router.