If you think your noisy phone line is causing problems with your modem, check the following:

How to check the quality of your phone line
Check the Line for Noise
Just what are these noises?
Check the Phone Unit
**ADVANCE USERS**
Check the Modem
Check Type of Modem

Check the Line for Noise
First you need to get rid of the dialtone so you can really listen to the line. Dial a number, (any number so that you don't get the dialtone replaced by a busy signal). The easiest and most common is a "1". When the dialtone is gone, listen carefully and concentrate before it times out and gets replaced by a busy signal again.
This is the first major step to checking if you have a good line for a modem or not. Is it quiet? No pops, bings, bangs, crackles, creaking, gsm interference (from gsm cellphones) or faint voices in the background from other people's conversations. All or any of these noises will cause you a problem on a high speed modem link.

Just what are these noises?
It can be something as simple as a loose screw on a connector block somewhere between you and the exchange.
Water getting into one of the Telco's cable ducts and the waterproofing (around the cable joints) has started to break down.
You could have a bad connection in the house somewhere, and not necessarily on the line extension that's feeding your modem.
Other people's conversations ("crosstalk/lines") are not so easy to pin down, and are usually more likely to be further "into the system" before the source becomes apparent.
A call or a text message is being sent or received from your or someone else's GSM cellphone too near your landline phone. Try to chose a spot for your modem and computer that is as far away as possible from any GSM cellphone communication. If you think you have determined that phone line noise is due to items 1, 2, 3, or 4 , call your Telco immediately to have it confirmed and repaired.

Check the Phone Unit
Next you need to check the phone unit itself. If it's just the phone unit causing the noise, it won't affect the quality you get from the modem. If you have a spare phone, take it and plug it into the same socket and then check for noise again just to make sure.

**ADVANCED USERS**

Check the Modem
Assuming that you are happy with the silence of the line (don't be a lamb about it). Now check the modem. How can you check if you have a problematic plug or socket associated with the modem? Well, surprisingly, it's reasonably simple but you'll need to be fairly quick in carrying out the next "test".
First, get the modem to go on line, but not actually dial out a full number (similar to what you did with the phone but using the modem instead). To do this you'll also need to have a phone plugged in, too, that's fairly near to where your PC/Modem are. Don't pick up the phone yet.
You're about to enter a command string to the modem, so you'll need to have the PC powered up and talking to the modem with whatever piece of communication software you use. In the case of Procomm, Smartcom, SmartTerm, Zterm, etc., it's simply a case of just firing up the software and getting to a terminal screen with the "OK" prompt from the modem.
(CONSULT YOUR MODEM OR COMPUTER MANUAL FOR MORE INFO REGARDING THIS MATTER)
If you normally use 1st Reader or Robocomm, then you'll need to fire up the Terminal Session for those packages and again just get to an OK prompt. Don't dial out a number.

Now type this:
"ATX0D1;"
(without the quotes) and what you should hear (if you have the modem's speaker on) is that the modem will pick up the line (dialtone heard), then dial a 1 and finally drop back to command mode (OK prompt on your screen) while holding the line open. The next bit is easy or more difficult depending on the accessibility of your modem's leads. i.e. internal or external modem. Now pick up the phone and listen.

THIS time, it's going to be your modem that may "time out" before your phone line does. Check the modem's lead, plugs and sockets by physically getting hold of them and just gently pushing and pulling on the lead at both ends. You'll find on these type of plugs that there is possibly a couple of millimeters of "play" even when the plug is fully seated in it's socket.
This is enough for our purposes. If there is a problematic connection, it won't take a lot of movement before it becomes apparent in the phone you are listening to. No noise? If your modem hasn't already timed out, drop the line by typing "ATH". If you did hear some crackling from that, then you'll need to get that lead replaced. It could be just the contacts of the plugs that have become a little tarnished. A good ink eraser with a fairly sharp edge should clean these up nicely.

Check Type of Modem
OK, you've carried out all the above and you are still getting poor high speed connects with your modem or it takes a really long time (two to three minutes) to logon. Now it's going to matter exactly what type of modem you have. You are going to have to consult your manual or ask your dealer for the specific information.

You are entitled to have this problem corrected:
When you call your Telco, make sure that you say there is nothing wrong with your normal voice communications (if that is the case...) otherwise they will just do a normal line check. Explain to them that you are using a modem on your line, you are getting poor quality figures from your modem. If you are on a digital exchange, ask them if you can have the AGC (automatic gain control) turned OFF and your line setting at the exchange, set to position "5". In most cases this should cure your connect problems easily.