I have an ADSL line with BT. It's ADSL in an FTTC area, since they offered it free of charge. My main connection is a Virgin Media Business Cable service, which is usually very reliable. Since BT offered ADSL free of charge on an this BT line I accepted, as a backup.
The issue is distance from the Exchange which results in around 5.9 - 6.5 Mb/s with upstream of 832 kb/s. This is with an SNR margin of around 9 dB. Over time the downstream gradually drops to around 1 - 1.5 Mb/s with the SNR margin rising to 15 dB or more. If I force a resync the original speed is restored.
I had thought that BT systems worked in both directions - to not only reduce but to increase sync speed as well? Clearly that seem not the case, yet the line is capable as a re-sync causes a higher sync.
Since it's a free backup, you get what you pay for, but it does seem to work just in one direction.
Earlier this year the BT Wholesale checker had an 'open order' on the line, which BT were able to tell me related to a wider re-grade of all ADSL lines that could switch to VDSL/FTTC lines at the same cost. Naturally the pandemic delayed those plans, but the new checker no longer shows open orders so wonder if these changes are still happening?