Question about my phone

So, I got a new phone, the Sony Ericsson w810i, I had another one the Sony Ericsson w___ I don't quite remember, but it was a flip phone. Anyway, both of them are on a pay as you go account. Now, my first phone, the flip one I bought 3 months ago, it came with free text messaging for a month, I ended up putting a $100 phone card on it, but then I stopped using it completely. Literally put it away for a few months and rediscovered it only weeks ago. Now, I bought the w810i and it is prepaid and it got free text messaging for a month. I ended up switching the SIM cards, putting the new one in the older phone and the older on in the new one. And I gave the older one to a friend. Now my question is. I'm getting free text messaging, and so is she! Mine, since it was in the older phone and the text messaging should now be on a pay per use, but it isn't. And hers is also getting free text messaging. I'm just wondering how exactly this is going down. Thanks