You can use any of the following (UNLESS ITS A MERIDIAN 1250 OR 2250 CONSOLE PHONE)**
OfficeRunner / Sennheiser wireless, Sennheiser Corded Headset Amplifier (UI770) / UI760 (DISCONTINUED), an M22 / M12 (DISCONTINUED) / Vista, or any corded headset system with a console phone that requires a 2 prong adapter (like a P10) you want to us a PTR/TPA/PJ327.
A PTR/TPA/PJ327 is a Plantronics two prong to modular RJ9 adapter. It costs $19.95 and will work with any Headsets.com, Plantronics, or GN Netcom product. The part # is 18709-01 ($19.95 U.S., $31.95 CAD).
The Customer will not have remote answer capability while using a two prong adapter so do not sell an HL10 with this!
**If they have have a Meridian 2250 or M1250: Only Use Plantronics H-Series headsets with P10/2250