I have an iDCS 8D keyset connected to a DCS 408 system. The problem I am having is that the volume on the handset speaker is very low, even at its highest volume. The person I am calling can hear me but I can barely hear the person.

I have tried changing several values in MMC 807, using the handset from another telephone on the keyset, connecting the keyset to other ports and finally resetting the system with default values. The problem persists. However, if I connect a DS-5038S keyset or a telephone to the system, the are no volume problems.

Does this mean that there is a compatibility issue with the keyset or is there some other MMC value that I should be changing? Could it also be a hardware fault and, if so, can it be repaired?

Sounds like the handset is faulty.
You've proven it by trying a different phone.

Is the low volume problem common with these keysets? What is the cause of it? Can it be repaired?