Based on my experience with Vonage, those must be the cases where the landline is transmitted over twelve miles of barbed wire fence.
These days, landline phone switches all turn your phone calls into bits, and it is my impression that most landline long distance companies just transmit the bits, so they'll all be the same. The difference is how much noise there is between your handset and the phone switch.
If you want really clear audio and don't care about the price, get ISDN service so the phone itself does the digital conversion and the link from your house to the telco is all digital and noiseless.
R's,
John