Leitner headset + an Easy Connect cord would only have compatibility switches in the headset part
Leitner headset with a Leitner Avaya cord or a USB cord you will have TWO compatibility switches (headset & qd cord)

Common known compats (If these compats do not work, encourage a quick experiment until you find the right ones)
Computer: 2 & 3 (Older QD cord also had their own set of compat switches [which should also be set at 2 & 3], but the newer ones DO NOT)
Aastra: 2 & 3
Alcatel: 2 & 3
Avaya: Headset - Unknown, QD Cord - 6 or 7
Cisco: 2 & 4 or 1 & 4
Cortelco: 2 & 4
Mitel: 1 & 3 or 2 & 3
NEC: 2 & 3
Nortel: 2 & 3
Panasonic: 2 & 3 or 2 & 4
Polycom: 2 & 3 or 2 & 4
Shoretel: 2 & 3
Yealink: 1 & 4 or 2 & 4
Zultys: 2 & 3
Grandstream: 2 & 4
All cordless phones: 2 & 3