I will try, but people already reported that although you can dial these codes with it, the carrier will reject it as invalid phone number.

See

https://bugs.maemo.org/show_bug.cgi?id=5357#c17

for how to properly dial USSD codes.
We just need a way to do this directly on the device without connecting it to a computer in PC-Suite mode. And I assume PHONET is the key.