BT landline phone customers could save £60 a year under new proposals being considered by Ofcom.
The plan would benefit over two million people that have taken out landline-only contracts with the telecoms giant, who could see bills slashed by "at least £5 a month".
Communications regulator Ofcom said relevant customers were often elderly, vulnerable, or those who have remained with the same landline provider for decades.
These customers are more often than not getting "poor value for money".
Ofcom found that BT's market power has allowed it to increase prices without much risk of losing customers, with rivals then following its pricing lead.
According to latest statistics, almost 80% of the UK's 2.9 million landline-only customers are with BT.
Ofcom said: "The cut would return the cost of line rental to 2009 levels in real terms, effectively reversing price hikes for landline-only customers.
"We are also proposing safeguards to prevent BT from making future increases to line rental and landline call costs by more than inflation."
Currently BT customers with only a landline pay £18.99 per month for line rental, but under the plans this would be reduced to £13.99 - a 26% decrease.
The regulator said customers of "bundled" services, packages including landline, broadband and/or pay TV, benefit from attractive deals, driven by strong competition.
But offers for landline-only customers have become "increasingly limited", with a number of providers withdrawing their telephone-only products altogether.
Sharon White, Ofcom chief executive, said: "Line rental has been going up, even as providers' costs come down.
"This hurts people who rely on their landline the most, and are less likely to shop around for a better deal. We think that's unacceptable."