No you can't add the SMS allowance without the data plan. The whole reason for the SMS 2000 Allowance is for pre-SP2 Exchange 2003 (including Exchange 5.5 and 2000) servers to have the ability to sync with your device. If you have Exchange 2007 and Exchange 2003, then it's far less overhead on data, server, the network, etc, if you sync with https push, than with text messaging.

Plus the txt message that was sent from the Exchange server to your device was used to initiate the connection, not to give anything other than a "Sync Now" command, at which point your phone connects to the server and sync that information over the Data Connection.

So without a data plan, it would just be a txt messaging plan, and couldn't be used with an Exchange server.

So if you want to sync with your Exchange 2007 and Exchange 2003 servers, you will need a data plan, otherwise you aren't going to sync anything...

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-Michael
Need help setting up your i730/i830 or other WM2003/WM5 Phone, as well as the other hacks I have done, check out the MRailing Guides and Links