Purchase IMEI unlock codes for the Nokia 6233 to allow it to be used on all networks.
A network lock, otherwise known as a SIM lock, is a control placed on the phone by the original network that sold it. They are able to do this because with the lock, they can then sell the phone for cheaper, offering the customer a better upfront deal. This lock does however stay with the phone permanently unless it is removed with an unlock code or software. Doing this is completely legal and won't impact your warranty in any way.
This does depend on whether or not you've selected to receive an unlock code or use the software, so I'll run through both.
If you have selected the unlock code, you will receive the code by email along with the instructions on how to enter it in the phone.
For the software, you will receive login information, full instructions and a download link. Following the instructions is simple and the software will then automatically unlock the phone upon completion, removing the need for you to manually enter it.