pacjt,
You could try increasing the Conference Gain.
Turn on Online Monitor under View at the top.
System > System Information [OLM Only]
-Conference Gain: 3dB of Gain
You may also want to make sure that your Audio Type is set correctly for DSCP L3.
System > Devices and Feature Codes > IP Connections
-Change Extensions for Processor Module and Expansion Card
--P6001 Processor Module
----Audio RTP Type of Service: 184 <-- This is expedited forwarding
----Data Type of Service: 184 <-- This is expedited forwarding
--P6101 Expansion Card
----Audio RTP Type of Service: 184 <-- This is expedited forwarding
----Data Type of Service: 184 <-- This is expedited forwarding
Then there is looking at what Codec you are trying to use for those phones. Try using G.729 to see if that improves it.
Also, if I remember correctly there are some noise suppression settings as well, but I can't remember where those are, maybe under each phone.
It has been a long time since I have been inside of a MiVO-250 so some of this information is a little fuzzy.
Thanks,
TE