Voice on cordless phone diminishes?

when the phone is answered the person i am talking to there voice fades to zero, but sometimes ok,purchased a new digital cordless phone thinking that the old phone was no good, but still the same, sometime it also hisses @ crackles i have done a line test and that is ok. i thought that having the pc on was effecting it but it still does it when it is switched off.are there any telephone enginners out there who can give me advice what to do. or any suggestions.just thought that this problem only seems to effect incoming calls, and not calls we make out.