Apple pie. Ensure your voicemail email feature is working. Then create a shortcode that when dialed from any phone will go to a VMPro module that does nothing more than prompt for an employee number, then sends an email to HR (or wherever) with "Employee $KEY just clocked in". The email header will contain the date/time info.
You could create a second shortcode to go to a second module that would be very similiar, for clocking out.
Neat idea - employees could dial these shortcodes from any phone, including a cheap pots phone hanging on the wall by the warehouse entrance or whatever.
Peter