DACS stands for "Digital Access Carrier System".

DACS is a technology that allows 2 telephone lines to go down a single copper pair and back to the telephone exchange.

DACS was a technology used by BT if a user required an extra telephone line and there was no full spare lines available or if there was a problem with another line and users had to share.

If you find you have DACS running on your telephone line then you are likely to have some problems when it comes to broadband. Unfortunately this is a problem that those who do have DACS on their line have to face and you may have to kick up a bit of a fuss with BT to get the issue solved. Even if DACS was installed before you moved into your house there can still be a problem.

Ofcom have a set of guidelines that could be of interest and useful to people who have DACS on their telephone line and want to know what they should to to get it removed.