On some devices this would be listed as "FXS port input gain".
And as Mango previously noted on his site in regard to Linksys:
In some situations, the default gain settings are inappropriate. This can cause audio to be distorted, it can cause echo, and of course cause audio to be too quiet. If this is the case, you can adjust FXS Port Input Gain and/or FXS Port Output Gain, one at a time, in increments of three. However, you should only adjust these if you are experiencing a problem.
Note: Input Gain = how you sound to the other party. Output Gain = how the other party sounds to you.
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www.toao.net/25-linksys- ··· gurationBUT---I cannot find similar settings in the Gigaset C610A IP menus.