I am not at all certain, but I believe the term "proph" (short for prophylactic) just refers to the very thin latex material used in calls. The "original" mouth call makers sought suitable material for making their calls and that just happened to be the thin latex used in condoms and for other medical purposes. That material is now generally considered to be around .0025 in thickness, but I am sure it can vary a bit.
Personally, I rarely use reed material thicker that .003,...and it seems my best calls are a combination of .003 and proph/.0025 or simply layers of proph. Of course, preference in call construction obviously varies with the user and their calling mechanics and abilities.