Incorrect Billing Prior to Receiving new phone

Please explain to me how is it possible that been on a BIS Blackberry package for over two years and having very low telephone cost that when i do an upgrade i get billed even before i receive the new phone and did not even do a sim swap? I was informed by the lady at our company that deals with our company phones on the 24th July that my phone is due for upgrade, on the 28th July i received the new phone at this point i am still on the BIS package on the 29th July the sim swap was done, i received the bill for my account on the 17 August, and noticed a high DATA usage charge for the 22nd July before i even knew that my phone was due for upgrade. An amount of R592.82 should i not have been billed on the blackberry package until the 28th July and there after once the sim swap was done be billed on the new package? I am so angry and confused as to how the billing was done. This happened two years ago and i did not take any action, i just migrated back to the BIS package and that worked fine i did not once have high telephone billing but now a few days into the new package and i get a surprise of this bill, i would like this matter to be looked at and resolved as i am very upset.