I'm thinking that it is voluntary, since
he uses his Magekyou to do it, and it is the same technique as Kakashi's.
Also because he is able to quickly change between using (to dodge an attack) and not using it (to attack someone). Furthermore, (spoiler if you still do not know Tobi's identity)
Naruto headbutted him and he did not dodge it.
Later, Naruto hits him with a rasengan, breaking his mask and revealing his identity as Obito. Although he was 'trapped' (because of the Bijuu-dama) on the other dimension, if it were automatic, I'm guessing he wouldn't have been able to not dodge it, being fried by the Bijuu-dama 'back here'.
Although I say voluntary, it may be seen as semi-automatic (or even automatic, for that matter), in the sense that he does not actually think about it, but rather acts instinctively. However, I think it is triggered by his will, and not entirely automatic.