How to do announced transfer with A190 paired to M900 using 3CX

We are using the following setup at a customer site:

• 3CX PBX (self-hosted, latest stable software)
• M900 Cluster (latest firmware)
• A190 headsets for call-center seats/agents
• agents can freely roam within the building which is important

Adding the M900 UA to the CSTA parameter list allows us to actually use the 3CX v18 web/desktop client to use the A190 headset for dialing.

The problem is: We really need the feature to do announced transfers. If someone calls, and the call is going to be transferred, our customer requires to announce that call before transferring it. But this seems impossible: The 3CX desktop client will only do blind transfers.

We previously used softphone headsets connected to the PC via USB, acting as a sound card. But this has proven unstable and unreliable. No matter if using DECT or Bluetooth based headsets, the range is very limited. Also, the software integration between the 3CX client and the headset software tends to break with every Windows update, volume levels change, and audio output devices are switched unexpectedly. So the A190 became a perfect solution and can even work independent from a PC, it doesn't inherit all those quirks.

According to 3CX, they will not give any support using the A190 as a CSTA headset. Forum research reveals that 3CX does not currently (if ever) support CSTA features of IP clients having multiple identities - which is clearly the case for M900 bases.

So here are our questions:

1. Is there a way to properly support the announced transfer feature by adjusting M900 settings? Does it survive updates/re-provisioning?
2. Can Snom partner with 3CX to add full CSTA support/integration for M900 devices, or at least add better transfer options? Currently, the 3CX parameters would be overwritten by 3CX updates.
3. How can we properly upload German voice prompts via the M900 firmware update feature? There's an input box to do that, but there's no documentation I could find which says what values should be entered.

Thanks for looking into it.