Call your voicemail box.  Press 2 to leave a message.  Enter your own
phone number as the destination to leave yourself a message.

When I tried this, as soon as I pressed 1 to send the message, my
Samsung A680 alerted me to the new message with a "New Message' screen.
 As long as this was showing, my pressing numeric keys was no longer
'heard' by the voicemail system.  I had to press the 'Done' softkey a
couple of times to dismiss the 'New Voicemail' screen, and then I could
once again command the voicemail system.

Sprin't voicemail system has lots of interesting features.  Listen to
all the menus.