ZyXEL Inbound Call Quality Issue

Telnet into the device (usually 192.168.1.1)
Enter your password
Select 24 - System Maintenance
Select 8 - Command Interpreter Mode

At the ras> prompt, enter the following:
voice config rtp index 1
voice config rtp packetsize 1 g711 3:20ms
voice config rtp save 1

Check that the settings were saved:
voice config rtp index 1

On other ZyXel phones on the page labeled "PHONE" on the navigation bar, change the "Voice Frames per Packet" to "Small" from the default setting of "Large" will accomplish the same.