I'm on an O2 iPhone Pay Monthly contract currently, and before that a SE phone also on an O2 contract. You can check your inclusive allowance online any time.
It used to be a nightmare to find the right page for your bill/allowances, but O2 made some changes shortly after the 1st Gen iPhones were released and it's much better now.
I didn't know about the 21202 number, but just tried it and it works! You can't send a "blank" text on the iPhone though - but a space worked fine.