For this, you’ll want the A10 cord, which is catalog # 1391 !
This will replace the QD cord that the amp came with if they’re still using a Plantronics or Leitner headset.
PS: If they’ve lost their pigtail cord, you can recommend the Headsets.com Connector Cord for Executive Pro Amp to Phone. This is Catalog # 2406. You can't find it on the website, but it appears in the product page in Filemaker