RJ11 6P4C to RJ45 8P4C Modular Phone Internet Extension Cable 3 Meter - AliExpress
V-max Cable Tester for RJ11 and RJ45
RJ45 to RJ11 Cable, 6 Feet Phone Jack to Ethernet Adapter RFAdapter RJ11 6P4C Male to RJ45 8P8C Male Connector Plug Cord for Landline Telephone - Walmart.com
Amazon.com: XCZZYB Phone Jack to Ethernet Cable Adapter, RJ45 8P8C Female to RJ11 6P4C Male for Landline Telephone Service (2 Pack) : Electronics