USB Tethering

I just checked, and DSL does include the "usbnet" module (in /lib/modules/2.4.31/kernel/drivers/usb/) for networking over USB. Because the DSL base system is quite old now, many newer drivers that work with usbnet are probably missing. In theory it should work with some phones [EDIT: actually for just Android phones, they should all do tethering the same].

Open a terminal window, type "ifconfig" and watch what network interfaces are listed along the left of the window. Now connect your phone in tethering mode and, after waiting for everything to work itself out, do "ifconfig" again and note if the new interface has appeared (most likely usb0, but maybe starting with eth). If it has, you can configure it using the netcardconf program in the DSL menu under "DSL>Setup>Net Setup", or do it manually from the command line if you're comfortable with that.

If it doesn't work, which isn't unlikely, look into the protocol that your phone uses for the ethernet over USB and check whether it is included with DSL, or if it's even possible to run with a version 2.4 Linux kernel. It it works in other Linux distros, you can compare the results of the "lsmod" command in the terminal before and after connecting the phone. The new modules loaded will be the ones DSL would need to load to get usbnet working.

Tons of (slightly outdated) information at this webpage (http://www.linux-usb.org/usbnet/). I haven't read it all myself. On that page, this application will be a "USB device running Linux".