Go to Configuration > Tuning Parameters and check AFE Line In Gain- Local Phone
The default value should be 4.
If yours is not 4, then set to 4. If it is already 4, then try a lower value 3 first, then higher values. Some phones have higher amplitude tones and require a smaller gain
This is the method to import Tuning parameters from the default file newtuning.cctx
If you have Comfigurator 3.1.1, and your Comfort ffirmware is 5.145 and above do File > Import Comfiguration Items > and browse for .CCTx filename newtuning.cctx in the folder Configs.
When the window Select Items to Import comes up, check the box Tuning Parameters and press OK
You did not say that you had a Comfort I which does not have these same tuning parameters
Try the following
Reverse the polarity of the telephone and see if it makes any difference
Change Location 100 value to 0 and reset (Eng Menu 7,4,1)
We encourage users of Comfort I to upgrade to Comfort II so that we can support them better, especially if you are developing software for Comfort. We have very attractive packages, see http://www.comfortforums.com/forum25/1403.html