It said those customers, who are often elderly or vulnerable, were not getting value for money, the BBC reports.
BT has nearly 80 per cent of the UK landline market, and Ofcom is hoping other providers will also cut prices.
The telecoms giant said it took its responsibilities “very seriously”.
“Unlike other companies, [we] have many customers on special tariffs for socially excluded or vulnerable customers,” BT said.
“Recently, we have frozen the cost of line rental for all of our customers who take a BT phone line.”
The planned price cut will effectively reverse cost rises seen in recent years, Ofcom said. BT customers, who pay £18.99 per month for a landline-only contract, would pay no more than £13.99.
The regulator’s chief executive, Sharon White, told the BBC: “We believe there are about two million elderly and vulnerable [BT customers] – strikingly about half are in their late seventies – and for this group of people their landline is their lifeline.
“It’s a group of people who have seen the cost of their landline bills rise by about a third in recent years, while BT’s costs have fallen by about a quarter.”
She said most people on bundled packages – which include landline, broadband and/or pay TV – can shop around for better deals.
BT is not alone in raising its prices. Major industry players have increased line rental prices by between 25 per cent and 49 per cent in real terms in recent years, while wholesale prices have fallen by about 26 per cent, Ofcom said.
The regulator launched its review of landline phone prices in December.