Epson WorkForce Pro WF-M5799 Workgroup Monochrome Multifunction Printer with Replaceable Ink Pack System - Manual (Page 229)

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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.
3. Connect a second phone cable to your telephone or answering machine and to the EXT port on your
product.