Ok, I think this is why delivery reports don't appear on debranded Three phones: supposedly delivery reports are disabled on USIM cards from Three, and that Three's firmware is the only thing that makes delivery reports possible. I think that Three has changed some setting in the GDFS, making the option disappear with generic firmwares?

Could someone analyse the default messaging settings in 3-mobile branded firmware and see if it would be possible to create some kind of patch?