What dumb phone provides tethering?
I haven't been paying attention to the current crop of dumb phones, but back when I was using them, it was pretty much a standard feature. E.g., Nokia 8290 had a v.32 modem and an IR port, and IR was pretty common on laptops of the day. Point the two at each other, and you can start a PPP connection to your dialup ISP. Then when GPRS data became popular, I had a Siemens M46, which didn't have an IR port, but if you got the data cable, you could plug it into a serial port and tether by setting up a PPP connection that dialed a special phone number (*99# or something like that). And my last dumb phone was a Motorola V195, which showed up as a serial port when you plugged in the USB cable, and again, you could tether by setting up a PPP connection. Actually, I think you could also tether over Bluetooth DUN, although I may be thinking of another phone.