In order to send SMS while on the phone using Sprint, do the following:
1. Press Menu Key
2. Select 'Conference Call'
3. Press Menu Key
4. Select Send SMS Text <any contact>
5. Scroll Up so cursor is over the name
6. Press Menu Key
7. Select 'Change Recipient'
I have a Blackberry Curve 8330 on Sprint running 188.8.131.52
Hope that helps!