Dell 3115cn Color Laser Printer - Manual (Page 350)

If you want to use your computer modem for faxing or for a dial-up internet connection, connect the
computer modem directly to the back of your multifunction printer with the TAD as shown above.
Set your multifunction printer to the Ans/FAX mode and set Auto Ans. Ans/FAX to specify the time for
the TAD.
Turn off the computer modem's fax-receive feature.
Do not use the computer modem if your multifunction printer is sending or receiving a fax.
Follow the instructions provided with your computer modem and fax application to fax via the
computer modem.
If you want to capture images using your multifunction printer and Dell ScanDirect™, send images
using your fax application with the computer modem.