This is a problem with the sent.ini file of Call Monitor
Shutdown Call Monitor
Right-click on the Call Monitor icon, select "Shutdown".
In windows explorer Go to C:\Program Files\Datatex\Callmonitor\PCname,
where "PCname" is the name of your PC.
Go to the sent.ini file
Right click on the sent.ini file and rename it (Example: Sent.ini and Todays date).
Open Call Monitor again.
The sent items list will be blank
Your sent messages will be deleted and a new 'sent.ini' file created for you.