that is a good question. I've encountered both forms. if it is a noun that is also a "suru" verb, you can, I guess, technically use with and without "wo", and probably it would even translate the same in English (though literally you would would be saying (i) I'm speaking on phone or (ii) I'm doing a phone speaking).
and what makes this all even more confusing is that if it is a noun that is not a "suru" verb, then you should use "wo", but Japanese, especially in some regions, are very lax about the use of particles and often drop a lot of them, including the "wo".