Customers could be in line for between £300 and £400 if the claim by the Collective Action on Land Lines (CALL) is successful.
A class action trial seeking £1.3 billion in compensation from BT for more than three million customers is due to begin.
Customers could be in line for between £300-400 depending on the length of their contract with BT if Collective Action on Land Lines (Call) founder Justin Le Patourel is successful.
After a series of appeals, the case is now proceeding to a full trial at the Competition Appeal Tribunal.
In 2017, watchdog Ofcom said that BT had unfairly charged millions of landline customers from 2015 after an investigation.
At the time, BT agreed to reduce the price of its landlines.
However, Mr Le Patourel took the issue to the Competition Appeal Tribunal as he believed the telecoms provider owed compensation payments, arguing that many customers were vulnerable and were owed refunds.
Mr Le Patourel said: “We believe BT has been systematically overcharging millions of customers over many years, and those customers could be owed hundreds of pounds each.
“Time really is of the essence. More than 40% of our claimants are aged over 70, and over 150 of them are dying every day. It really is vital that BT should refund every one of them as soon as possible.”
A BT spokesman said: “We take our responsibilities to customers very seriously and are dedicated to keeping our customers connected, while helping those who need it most.
“This claim relates to a technical landline pricing issue which was resolved by Ofcom in 2017.
“We do not accept that our pricing was anti-competitive back then, and as such are committed to robustly defending our position at trial.”
