Some people pronounce "Ubuntu" as "You-bun-too" (or thereabouts), and if they were correct, then it would be "a Ubuntu phone", just like you would say "a yellow phone", not "an yellow phone".
However, my exhaustive research (a whole two minutes worth) leads me to conclude that "Oo-boon-too" is the correct pronunciation (note, this is how I've always pronounced it, so I may be biased). In that case, "an Ubuntu phone" is correct.