I use a Grandstream GRP2616 SIP deskphone—I’m a lawyer after all, and teleconferences are no fun on a cell phone when working from home. When I put stuff away during the pandemic, I lost track of the RJ9-to-QD cord that allowed an analog Jabra Biz 2400 II QD (quick disconnect) headset to work with the phone. The Cisco-compatible Jabra cord, 8800-00-37 that I had does not work on the Grandstream phone: I could hear, but the mic was clearly not transmitting.
There are forums online where people complain about the same issue. It’s because all the companies vary in how they utilize the 4 pins of the RJ9 cable.
Grandstream’s outdated compatibility charts point to the GN1200 smart cord, but did not say which setting to use. The GN1200 cable has a dip switch for those various headset cabling standards, and it turns out that I needed setting 4 (“Japanese phones, NEC, Nitsuko”) which has the M+ and M- wires reversed from the standard setting. This works with both the earpiece (for listening) and the microphone (for talking).
