Always wondered what the "prior" refers to. Prior to take-off? To joining the circuit?
For those thinking it ridiculous: the one time I had NOT called before take-off, I found the place rented out for some commercial event and the runway cluttered with booths and people. Of course I had a plan B, there was never a real problem, but I had no plan C so I was on the lucky side.
Edit: over here, many aerodromes explicitly require "PPR by telephone" - check
Recreatief Vliegveld Grimbergen - EBGB - Grimbergen Airfield for just one example - and since we're supposed to NOT use our mobiles while airborne there's few options left.