I dug it out to check if I could get that working under Unix.
The problem I ran into way back when was that the WiFi driver could not switch the USB stick to Master Mode (read: become an access point). Now, I found out
how to do that. Hooray for me!

Of course, I’m not there yet – the phone can find the network, but now it still needs to forward everything from its normal Internet connection (i.e. bridge ppp0).

To prevent failure in case of linkrot, below is a recap of the partial solution provided by the above link.

You need:
– sudo apt-get install hostapd
– sudo apt-get install isc-dhcp-server

# turn the wifi off - can also just click "Enable WiFi"
sudo nmcli nm wifi off
# deblock Wifi with rfkill
sudo rfkill unblock wlan

# bring the wifi back up & wait a bit
sudo ifconfig wlan0 10.15.0.1/24 up
sleep 1
# start the DHCP server
sudo service isc-dhcp-server restart
# start the Access Point server
sudo service hostapd restart

Remaining challenges:
– get an IP (somehow not working)
– setup WPA2 (if I try, I get authentication errors)

Edit: the main challenge is that I’m using a 3G usb modem as my internet connection. That modem does not speak IP (but ThreeGeese). So standard bridging solutions don’t work – the packets arriving at the computer are not in ThreeGeese and thus cannot be forwarded.
Pointers to investigate:
– https://help.ubuntu.com/community/SharingMobileBroadband
– http://ubuntuforums.org/showthread.php?t=1822957

To prevent the popup message from appearing at random times, go to the Messages app, settings -> MMS, and uncheck “auto-retrieve” (or similar named setting).