The telecoms regulator is looking into BT because there are suspicions that it violated the law when customers signed new mobile and broadband contracts.
The telecoms behemoth is being investigated by Ofcom to see if it failed to give customers the "clear and simple" contract information that it was supposed to.
It claimed to have information showing that the rules had been broken by two BT subsidiaries, EE and Plusnet.
With Ofcom's investigation, BT claimed to be "fully engaged.".
Before signing up, customers must receive contract information from telecoms providers as well as a brief, usually one-page, summary of the key contract terms.
Since June of last year, there has been a rule in place. The adjustment was made to help prevent people from being caught off guard by unexpected price increases at a time when household budgets are already under considerable stress.
Key details must be included in the summary. It covers details like the contract's length, price, broadband speed, and the terms and conditions if a customer chooses to break their contract early.
The documents are also available in accessible formats upon request from customers with disabilities.
According to Ofcom, the regulations are a crucial tool in enabling consumers to confidently compare prices and make the best decision for their needs.
The regulator claimed it has "reasons to suspect" that Plusnet, a BT subsidiary, may have "failed to comply with these requirements.". ".
The regulator began an investigation into EE, a member of the BT group, in October on the grounds that it had reason to believe the company had broken the law as well.
As a result, Ofcom stated: "Because of the suspected violations by each of these subsidiaries, our investigation will now consider whether BT has violated Ofcom's rules.
As our investigation develops, we'll gather more data and let you know. ".
"We want our customers to be fully informed, and we make sales information upfront, clear, and transparent," the BT spokesperson said.
"We are actively cooperating with Ofcom throughout this investigation.