Telephone Training Adapter Y Splitter with RJ9/RJ12 Plug Connections for Corded Headset or Handset for Aastra Nortel etc phone - AliExpress Consumer Electronics
Amazon.com: DB9 Serial RS232 to RJ12 6P6C Adapter Cable for APC PDU 940-0144A : Electronics
DB9 PEDALS T.RJ12 ADAPTER
Modular Voice T Adapter - 1 Male to 2 Female (RJ11 - 6P6C for 6 Wire) | Computer Cable Store