The Automated Attendant is answering the call initially, and when the caller doesn't dial an extension number, the call is timing out and transferring to Ext. 10, where the call is also not answered, which covers to Mailbox 10, which has the default message of "Your party is not available".
If you don't want the auto attendant to answer first, and give your callers the option of dialing a specific extension, then change your setup to have mailbox 10 cover the incoming lines, and record Mailbox 10's personal greeting to say "Thank you for calling XYZ Company, please leave a message at the tone."
If you want the Auto Attendant to answer first, record the AA greeting to say "Thank you for calling XYZ Company. If you know your party's extension number, please dial it now, or to leave a general message, press 0 or simply stay on the line". Then record Mailbox 10's greeting to say "You've reached our general mailbox, please leave a message at the tone"