Grandstream 1st second of audio cut off

Having an issue where callers to a Grandstream phone have the 1st second of audio cut off. Test methodology:
Call a Grandstream phone (I am testing models GXP1625, GRP2614). Immediatedly have the person on the Grandstream phone start counting verbally and quickly. Usually the 1 and maybe the 4 or 5 counts are cut out to the other end. After that the call is good.

Testing Notes:
FusionPBX 4.4.7, Freeswitch 1.8.7
It does not matter what network this is on nor firewall settings. The problem occurs on the local LAN between phones, to a routed network, and from the outside PSTN.
The problem does NOT occur with a Fanvil phone as the called phone (no Grandstream), nor using a softphone (MicroSIP).
Server Hardware is fast with virtually unlimited resources. The problem occurs with FusionPBX installed on physical hardware or virtualized.
The problem occurs no matter what codec is used -- have tried every codec/setting in the audio section of the Grandstream settings.
Firmware for each tested Grandstream is the latest as of this writing.

Anyone else encounter this, have any ideas?
I had a similar issue with a Yealink W52p, the customer would answer the call but need to say "Hello" 3 or 4 times before the audio started properly. I captured several packet traces and these all showed the RTP streams starting immediately on answer.

It turned out to be a firmware issue with the phone and a firmware upgrade solved it. BUT the issue must have been caused by a combination of factors because I tested other W52ps at the same firmware revision but they did not exhibit the fault.

FYI if it helps anyone else been working with Grandstream on this issue. After numerous packet captures and beta firmwares, Grandstream has fixed this issue for the GXP1620/1625 series. The fix is in the latest firmware Release_GXP16xx_1.0.4.152

Still having issues with the GRP series, hope for a fix soon.