Snom D7 Expansion module
The USB hub inside the D7 expansion module has a hardware limitation, preventing using a headset and cascading other D7 expansion modules at the same time.
By default, the USB stack is configured to allow daisy chaining up to 3 modules. In this configuration, connecting a USB headset won't work correctly (no audio).
To circumvent this, a new setting has been introduced in firmware version 126.96.36.199: enable_ehci
When this setting is Off, audio USB headsets will work correctly, and modules daisy-chaining won't be possible anymore.
=> Please note that a reboot is needed to apply this setting
In a scenario where both a USB headset a multiple D7 expansion modules are needed, it's possible to use a standard USB hub connected directly to the phone, with D7 modules and USB headsets connected to it.
Snom D3 Expansion module
The USB hub element inside the D3 expansion module is able to handle both a USB headset and multiple D3 modules at the same time.
This also requires the enable_ehci setting to be Off