Can anyone tell me if this model phone has a full duplex speaker? I had a customer call me saying the volume goes up and down while using the speaker. It sounds to me as if they are only half-duplex, but I can't find any info on it.
Vertical SBX IP 320 4024-00 24 Button Digital Phone

No, it's not full duplex.

There are plenty of gain adjustments that can be made in PGM400 and 402 that will help you to fine-tune the levels. I think you'll be happy with the performance once you adjust these to match your specific environment. Just remember that you have to do a soft restart in order for these changes to take effect.