The built-in USB hub of the D7 expansion module, with a hardware revision < R4, has a hardware limitation, preventing the usage of 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 to a D7 won't work correctly (no audio).
To circumvent this, a new setting has been introduced in firmware version 188.8.131.52: enable_ehci
On these phone types, the setting enable_ehci can have two possible values:
On a D785
On the D785 the setting enable_ehci is not present and therefore also off, as this model can't handle ehci. Daisy-chain is not possible, but USB headsets plugged into a single D7 expansion modules connected should work correctly.
If you have a white D7 or a black D7 of hardware revision R4 or higher, using this as the first device in the chain, will solve this issues explained above and support to use older hardware revision D7 models behind it.
Again, the setting enable_ehci must be set to off.
Similar to D7 white and new D7 black R4 or higher, 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. As this requires the setting enable_ehci to be set to Off, permanently, the setting is removed from the D3xx series.