Kyocera QCP 6035 Reference Manual page 196

You may also see "Failed to Initialize Modem" if you attempt to make a modem
connection while you are in a voice call. You cannot make a voice call and a data
call simultaneously using the same modem.
When Data/Fax mode is in the "Modem Ready" state, you can make and receive
calls on the phone. Once it is in the "Modem InUse" state, you can no longer
make or receive voice calls until you end the modem session.
You cannot use other applications while running Data/Fax mode. If you open
another application, your modem session automatically terminates.