Izanagi. I believe it was stated
it wasn't stated ^_^
Because of dumb plot.
He could have Kamui teleported anywhere.
He phased part of his body to let the attack go through him so he didn't Kamui his whole body into the other dimension.
He only phased the necessary part of his body to avoid the attack ,

as proven in the Minato fight , he has to become completely physical again in order to use Kamui . In other words, if he phases, he has to undo the phasing ability in order to use kamui.

Konan had multiple explosive tags in place going off to explode on Obito so he couldn't just undo the phasing ability to resort to kamui because he would be blown to pieces due to the chain reaction of explosives (similar to Tobirama's explosive combo) too many explosions occurring on his body to stop him from being able to use other jutsu.
He can only stay phased for a total of 5 minutes and Konan rigged enough explosives to go off for over more than 5 mins.
She hoped his phasing time limit would run out and he would be forced to undo it and take the force of the explosions.
He did and he had Izanagi as a back up so he could undo his *death* due to damage sustained from the explosion. xD