For example, modems capable of 33.6k often will connect at only 28.8k on calls from one telephone exchange to another, because many of the trunk circuits that carry calls between telephone switches can't carry calls with the fidelity needed for 33.6k connections. While these older trunks are digital, they only guarantee 7 out of every 8 bits. Human hearing cannot detect this difference in fidelity, but modems trying to push the telephone network to the absolute limits can. They detect the slight loss of fidelity and back off on the speed to what the circuit can support. Since the phone network is a mixture of newer and older trunks, the digital fidelity you get may vary from call to call.
People living in some suburban housing developments can have a similar fidelity loss.
Some of these remotes only have fidelity adequate for 26.4k modems connections. If you get solid connects but always at 26.4, ask your phone company if your phone service is via an SLC remote. Severe or unstable phone problems, the kind that cause disconnects or failure to connect at all, are nearly always problems with the wiring right in user's homes. Many of these problems come from not knowing the special needs of telephone wiring and treating telephone wiring like electrical wiring. So let's look at why good telephone wiring takes special care you wouldn't expect.