Only individuals skilled in lucid dreaming can communicate directly with a Tulpa. If a person is not skilled in lucid dreaming the best they can do is a vague interpretation of normal dreams they have that may or may not be true. For instance, how would I know a dream about an important conversation with a tulpa wasn't actually just purely a construct of my subconscious that had nothing to do with actually contacting them?
So I begin training myself in lucid dreaming so that I can differentiate between a dream I actually control and one that is just a run of the mill dream. Eventually I get good enough that I can just ask a tulpa something that I couldn't possibly know to verify that the contact is real then test it during the day. A possible test might be to have my wife hide my car keys somewhere while I'm out for a walk before bed so there would be no possible way my subconscious could dredge up the answer into my dreams. I ask the Tulpa to tell me where they are, then wake up and find them in that exact spot. Okay, now i know I'm actually in contact with a real Tulpa and that it is communicating real information to me.
Now, Tulpas have names, which act kind of like the magical version of a web address. If a Tulpa wishes to let me remain in contact it gives me that name so I can call it up whenever I'm lucid dreaming to consult it about whatever I need to. Obviously its not an exact science since Tulpas have no binding contract to be honest but its now a lot more accurate than regular dream interpretation that could just turn out to be assigning meaning to random subconscious gibberish.
In this way you add some fun variation in powers to your story, since lucid dreaming is something that can be easily, regularly, and naturally accomplished without effort by some, and a matter of lots of training for others, or even a combination of both. You combine users with natural talent and people with training and people who just cant quite seem to get it to work.