Epson WorkForce Pro WF-C5890 Wireless Color MFP Inkjet Printer - Manual (Page 212)

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2. If you are connecting a telephone or answering machine to your product using a second phone
cable, remove the cap from the EXT port on your product.
Note: If your telephone has a fax function, disable the fax function before connecting it. If you are not
able to completely disable the fax function on your telephone, you may not be able to use it as an
external telephone.