This usually means there's either an issue with your micro-filters, your phone, or a fault with the line. We've covered below a few steps you can try below.
Check your micro-filters
Do you have any sockets that look like the 3 below? If you've got anything connected into them make sure there's a micro-filter plugged in first.
Your master socket might look like either of the 2 below. These have a micro-filter built in so take any micro-filters out of this socket.
Make sure your micro-filters aren't broken
Try your phone connected into the same socket as your router and disconnect everything else. If your master socket needs a micro-filter make sure you're using one. Ideally try and use a new micro-filter or one from another socket for this test. See How do I check or change my ADSL filter? for more information.
If this fixes it chances are you may need some new micro-filters (available from BT Business Direct).
Try another phone
Try a different phone on the landline. If you've got a corded phone these are ideal for testing but any phone will do.
Nothing worked? Try our online diagnostics
Go to our Repair Centre where you'll be able to identify, fix, report, and track your fault online. It's simple to use but effective: we'll take you through a series of steps, run some tests on your line, and (if we can't fix the fault there and then), we'll arrange for an engineer to come out to you (where that's appropriate).