Echo problem with Jabra GN1900 Headset

[GHEH]

I put the handset on the headset input to check if the handset had echo problems on the headset input.

But no Echo. So the headset input seems ok.

So a short summary:

Headset on handset input -> No echo
Headset on headset input -> Echo
Handset on handset input -> No echo
Handset on headset input -> No echo

So it's really hard to say what the problem is isn't it?