Had a look at the aircraft maintenance and schematics manuals and the only difference between the handset and ACP’s is the handset goes directly to CIDS whereas the ACP’s all go to the audio management unit then from the AMU to CIDS.
It’s the CIDS that receives the engine running and cabin decompression signals to increase the volume so the same volume would be heard from either source of mic (Boom-mic or Handset)
The ACP 1, 2, AMU and CIDS are all powered by the 28VDC ESS BUS and the ACP 3 is powered by DC BUS 1 so in theory they are all always available.
//Edited to replace BATT BUS 1 with DC BUS 1//
Last edited by aeo; 18th Apr 2022 at 05:59.