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.

Adrian.