The process of sharing the internet from your phone to another device like what you are doing is known as tethering. In your case you're making your phone appear as a dial up modem.
In Archos' case, you have some options. I don't know, but I don't believe you can tether an archos to your phone like that. I could be wrong.
As for why some phones fail to tether, it could be a couple of reasons. The most common one is that the phone is putting out an Ad-Hoc wifi signal. All android devices cannot pick up Ad-Hoc (peer2peer) signals, only Infrastructure (Access Points, Routers). There is another option for Bluetooth tethering, which I've gotten to work off my Motorola Droid, but other phones have some issues. I believe its a matter of what bluetooth capabilities the phone has (bluetooth specs similar to things like wireless audio transmitting).
I hope I cleared somethings up