Why only AT&T users cannot listen my webradio

[lsmmello]

Hi! Please, I'm a newbie here. So I beg patience
I'm using Winamp Shoutcast plugging and listen2myradio servers to stream a webradio.
Everything is working great, except one thing.
Everybody can access and hear well using:
- Any web browsers on a computer,
- Any android smartphone with flash palyer or a shoutcast app,
- Any iphone from other carriers than AT&T.
The AT&T users can hear only over WIFI but not using the AT&T network.
What happen with them is:
The listen2myradio page loads, but the play button never loads. Only a black square (where its supposed to show the play button) is showed.
What I'm doing wrong? Is a matter of configuration?
Thank you!