I tether my A5IT to my Nokia N97. I can also tether my laptop to my N97, either by USB or by Bluetooth.
On my laptop, in my Network settings, against the N97 connection my laptop shows "Phone# or Host Address".
The entry for both USB and BT connections is *99#. This setting comes from the Phone Operator.
The Archos, tethered to the same N97, asks for the Access Point Name, which, in my case, is "mobile.o2.co.uk" - my Operator is obviously O2.
So the 2 devices accept different forms of accessing the Access Point.
I just tried changing the Archos APN to the (phone number) *99#, and it wouldn't tether. On my Archos at least it wants the "Host Address" form of the APN, i.e. the Operator's GPRS ip address.
This link gives UK ip addresses for the major operators, although it may be out of date:
http://www.filesaveas.com/gprs.html
Maybe there is something similar for the US and other countries?
EDIT:
Do you not have on your phone somewhere, the details of the phone's GPRS Access Point? That's where my "mobile.o2.co.uk" is set, and from where I read it in order to setup the Archos.