Do you see any packet loss in call status?
What is packet loss resilience configuration on your MX300?
=====================================================================
xConfiguration Conference 1 PacketLossResilience Mode: On
xConfiguration Experimental PacketOverloadHandling WhenDetected: Unencrypted
xConfiguration Experimental Conference 1 PacketLossResilience ForwardErrorCorrection: On
xConfiguration Experimental Conference 1 PacketLossResilience RateAdaption: On
=====================================================================