BT's PSTN (voice) lines, as used by most residential customers, can cause some serious problems when connecting to Internet Service Providers (ISP). That has no bearing on the Internet or the World Wide Web, as these are services provided outside the Network.
BT's PSTN network is analogue (voice) and BT makes no provision for any form of service for digital use, period ! So why is everyone promoting the connection to ISPs on normal Voice lines ?
In fact, why is everyone avoiding informing the customer at the point of sale ?
To make matters worse, if a DACS unit is fitted to a PSTN line which doubles the capacity of a single line, problems will occur, especially if you have a longer distance to connect to an ISP.
The DACS units are only designed for 'Voice' (analogue), and are known to be unsuitable for digital use. Under the current License issued by Oftel in 1985, BT can, and do, make no guarantee 'Whatsoever' on their PSTN Network for digital use and any provision for, service or quality. It is strange that despite this, members of the public are not being informed of this fact, especially when purchasing those type of devices. It is worth pointing out, that BT's own shops are selling these devices without notifying anyone at the 'Point of Sale'. Indeed, their advertising with regards to their ISP totally avoids the topic !
Other companies, in general, who are also selling digital devices to be used on PSTN lines, are totally oblivious of the facts.
Digital devices are being sold as BT 'Approved'. That is not the case. BT 'DO NOT' make such approvals. Remember, that does not mean that such devices will work on the PSTN network.

BT PSTN customers who are having trouble connecting to ISPs, should contact the 'Fault' department on 150 and find out if they are on the DACS unit and when it was fitted. Oftel has informed me that BT MUST inform you. If the problems occurred at the start date of the DACS installation, then it is quite possible that the DACS is at fault. Your telephone bill will show you the terminated call and charges. Customers may request to be removed from the DACS unit and placed on to a normal PSTN line. If you are unhappy with the result, contact Oftel, your Trade and Standards Officer and your Local MP. It is very important that you do this.
Just one more interesting point to note. If you are on the PSTN network and connected to a DACS unit and no other spare line is available to give a clean unDACSed line, then you are unable to have the digital 'Super Highway' or a 'ISDN' line. When asked why? I was informed that the DACS unit was unsuitable for digital use.
Funny isn't it ? I do not think so !
BT are fully aware of the growth and demand on Internet products and connections, considering they are the main suppliers for 'lines' to ISPs and the general public here in the UK. So why don't they update the PSTN Network to ensure it operates normally on digital and scrap the 'analogue'. After all, most of the PSTN network has 'copper' cable which does handle digital (53k) perfectly ok. Offering special 'Digital' and 'ASDL' lines to normal working people at prices that most could not afford, is quite frankly 'an insult'. Whats more, it restricts the access to the a FREE 'Internet' service for those who have now where else to purchase quality 'line rental' products and services. It's time Oftel stopped messing about and reviewed the 'licence'.

ISPs: From 'Paying' to 'Free'. Select HERE.

Some good advice.
A). I advise that everyone gets as many FREE ISPs as possible (0845 and so on). This will help you when testing out your modem connections. Providing that your computer system is not slow, then some interesting points should be noted....
If you can not connect to any of them, its highly unlikely that ALL ISPs have wrong settings. It's more than likely the Telephone line. If you get a connection and the speed is very much slower than it should be, then you can start getting to grips with your Telephone Line provider. The very first thing they will say is "It's your Internet Service Provider". Do not stand for it. Most ISPs have sorted out connection problems early on in their 'startup' program (there are exception). The chance it's them is remote to say the very least. Providing your modem is installed correctly and working, the ISP Dial Up Network (DUN) settings are correct, then most speed and connection faults are with the telephone line.
B). Some people are giving the advice that one should ask BT to 'turn up the Gain' on the telephone line. This will NOT work in any respect if you have a problem line. All it will do, is to increase the 'Gain' on the problems present, thus making it worse ! Sort the 'Line' out first, then see if you can 'tweak' to give faster connections.
C). I have heard that there are a couple of companies selling DACS units for the public to connect to their line, thus splitting the line into 2 individual user lines. DO NOT PURCHASE these unless you know the FULL 'ins' and 'outs' of those types of products. You will not be able to use them correctly on BT's PSTN (Voice) Network, if at all, especially if BT's own DACS unit is fitted to your line. Connecting to an ISP may be very difficult indeed, if not impossible. The question also arises, who supports these devices and more to the point, which ISPs ?
D). I advise strongly that you contact your BT Local Area Complaints Manager as soon as possible if the matter has not been resloved in a few weeks. He or She may be able to get things done very much faster. Do not give up!
E). Where possible, get things in writing.
F). And Finally. If you are on BT's PSTN network, it may not be to your advantage to join one the the 'paying' schemes. If you already have constant trouble connecting, its just another unwanted expence.