VEC TRX-20 Telephone Call Recording adapter with 3 ft. cord and 3.5mm plug. - Walmart.com
Amazon.com: Telephone Line Adapter for Split the Line to Telephone and Voice Recorder, with Two RJ11 Port for Incoming and Outgoing Phone Connection and 3.5mm Audio Socket to Connect to Voice Recording
Telephone Recording Jack Adapter Attachment Installation to Record Phone Calls - YouTube