I need help with blackberry messenger?

basically i have had a bb since december and i paid �5 for it once and ive been getting it free all this time. Im on 02 pay as you go and i asked an advisor and they said this isnt possible. i have a normal bb bolt on. Is this a glitch? i havent topped up since i got it too and im gunna do it tommorow and if i do it, do you think the free service will go? HELPPP i asked like everyone, help is appreciated :)