BRL - I've checked again:
"ADSL enables fast Internet to be delivered to you on your standard telephone line. It allows you to surf the Internet, use e-mail or many other internet applications whilst still carrying on using your telephone, answer machine, fax or any of other piece of telephony equipment that you have attached to your line.
To ensure the quality of both the telephony and the ADSL every piece of telephony equipment must be filtered including telephones, fax machines and even simple tone callers. This does not mean that every piece of equipment must have a filter, only that every telephone socket that has telephony equipment attached to it must have a Microfilter fitted first. A standard telephone doubler can be used in the Microfilter “Phone” socket to enable you to plug in another telephony device or extension cable."
So you really do need to change the way you've got your system connected if you're going to use ADSL without any problems.