Well, you can simply access other applications when on the phone without cutting your caller off. How you do it, however, depends on the device model.
For older BlackBerries select simply select "Home Screen" from the menu while on the call by hitting the track wheel. Then access the applications you wish to use.
For the BlackBerry 8700 Series and BlackBerry 7100 Series devices, press “Escape” to reach the applications.
For the 7250 hold “Control” and then click the “Escape”button.
Then press the phone/send button or select the phone application to return to the Active Call screen.
If you are on a call and need to jot down a number or name quickly you can use the "Notes" feature of your newer BlackBerry. (OS 4.1 and later)
During the call, press the track wheel and select "Notes" from the menu. Type what you need to save (numbers or name or whatever you need to remember) press and from the menu select "hide notes." When the call is complete highlight the last call in the phone menu, press the track wheel and select 'View History'. You will see the tiny note icon on that indicates you made a note. To view or edit select and open 'edit notes'.
For other OS versions, simply press the ALT key and press "Escape." Then select "messages," "home screen" or wherever you need to access during that call for the info you need.
You can also use this function while using other applications. For example, you might be writing an e-mail and need to access your calendar. Follow the same procedure as above.