Any or all of these can cause just enough noise in a phone line to make modems connect at unacceptable speeds of 24000 or less. If there is any question, have a professional inspect the jack and line in the house. Most telcos will do this for a minimal fee. Telcos also are required by law to check the quality of your line, so this is the second place to examine. The culprit in this instance is usually line noise.
Line noise is the slight crackle or hum that be heard when talking on a telephone.
Even if th e noise is present and can not be detected by the human ear, modems will definitely pick this distortion up, and fall back to lower speeds than desired to find a comfortable transfer rate. Another place to check when it comes to telcos is the switch to which an entire neighborhood of telephone lines can connect before being routed to an actual telco building. If the switch is out-dated, overloaded, or has a bad card or some other hardware malfunction, a modem will be affected. This type of problem is probably the hardest to correct, because getting a telco to replace part of a switch is an expensive and time consuming process. The best way to take action in this situation is to have friends and neighbors in the area all call the telco and complain.