The telecoms regulator will review the price of standalone landlines which are purchased without broadband access or pay-to-view TV over concerns the elderly and vulnerable are being hit hard by rising costs.
Ofcom said all major landline providers had increased line rental charges by between 28% and 41% in recent years, despite the wholesale cost of providing the service falling 25%.
Market leaders BT and Virgin Media had the highest line rental charges, with TalkTalk and Sky not far behind, according to Ofcom.
Virgin Media today announced it is freezing line rental prices for phone-only customers who are disabled or aged 65 or over though it insists this isn't in response to Ofcom's announcement.
Ofcom said line rental price increases were felt most acutely by customers with a standalone landline service who do not benefit from strong competition for bundled packages including landline, broadband and/or TV.
Ofcom found that elderly and vulnerable people who make up a significant proportion of standalone landline customers in the UK were particularly affected by price rises.
They were often "very reliant" on their landline and more likely than most to have stayed with the same phone company all their life.
Ofcom's competition group director Jonathan Oxley says: "Our evidence shows that landline providers have been raising the price of line rental, even as their costs have been coming down.
"We're particularly concerned for older and vulnerable customers, who rely on their landline and are less likely to change provider. So we're reviewing this market to ensure these customers are protected and getting value for money."
Having announced its review today, Ofcom expects to publish its findings early next year.
To find the cheapest landline deals, check out our Home Phone Guide.
Additional reporting by the Press Association.