Quality of uniden phones

I have a set of 4 Uniden cordless phones. The quality is terrible on some of the phones--and it's not always the same phone. I can't tell, of course, when I'm talking, and most people are too polite to tell me that the quality is horrible. If I ask, they'll say it's bad. The phones are a year and a half old and they've done this since the beginning.

Also, how do you turn the volume up or down when you're speaking or listening?

Thanks!