I recently had a case where my line was constantly disconnecting, then reconnecting but no data went through. Tested 3 modems and 2 ISPs before going to Telkom. They reset the port and ran a line test and said everything was fine, but I still had the same problem. Then it got worse, the modem synced at a lower speed and my landline was off completely. I phoned Telkom 3 times to make sure they did a proper test on the line. What I did notice was that my router (at the time it was a Linksys) showed an outrageously high number in the line stats, downstream margin (a noise value if I am correct). I tried to explain that to the Telkom techs but their first line knows as much about technology as I know about open heart surgery.

Long story short, on the third day after my first call, I picked up the phone and it was working again. I checked my modem and it synced at full speed, no more disconnects. My line has been stable since. I am pretty sure it was a faulty port on the DSLAM or something to that effect. The technicians in my area are good, the obstacle is usually Telkoms hotline.

We also have a DSL reseller facility for our company, and I sometimes do basic diagnostics when a branch reports that they're down. Something I often see as reason for disconnects are Port Errors, which usually indicates a Telkom problem. Often it is enough for them to reset the port, but sometimes they have to go further than that.