Using ATA gateway devices to connect analog phones, fax machines via FXS.
Grandstream ATA HT801 - Onedirect
Amazon.com : Yeastar YST-TA800 8 Port FXS Analog Telephone Adapter VoIP SIP FAX QOS Gateway : Office Products
Distybox 300 adattatore per telefono universale adattatore per telefoni analogici fax apparecchi multifunzione DECT/GAP stazione base o router : Amazon.it: Elettronica