I seem to be having intermittent microphone issues where the person being called cannot hear the caller. What things should i be looking at to trouleshoot? Thanks in advance Chris

We have a Akuvox R28 system on our building and it curretly has firmware version 28.31.1.137. I'm looking to upgrade the firmware but it says the file is too large when I try to upgrade.28.30.4.3 is the latest version.