I had been experienced all kinds of poor-quality calls before like delay sounds, chopped sounds, one-way conversation before from time to time mostly during evening time. I found out that most of the time that poor calls made, I and/or my callers used the SPEAKER PHONE feature. I have stopped using that feature since and all the poor calls dropped by 90%. The solution would work better if my callers also turn off their speaker phone too (When my callers told me they did not hear me well, I often advised them to turn off their speaker phone then the problem normally solved). Hope that this little trick would help others too.
By the way, I am using a cordless Panasonic 6.0 DECT system.