Grandstream recommend the following:
It depends on what codec you choose and balance between bandwidth utilization and impact of packet loss. The bigger this value, the higher bandwidth utilization because more voice frames are packed into the payload field of a UDP/RTP packet and thus the network header overhead would be lower. However, the impact of a packet loss on perceived voice quality will be bigger.
For PCMU/PCMA, the default is 2 and max is 10
For G723, the default is 1 and max is 32
For G726-32, the default is 2 and max is 20
For G729, the default is 2 and max is 64
For G728, the default is 4 and max is 64
We have found that if you are using G711 codecs you may find that you get a better quality of call if you set this setting to 1.
Please experiment with this setting.