Regulator Ofcom is investigating BT to see whether it broke rules when customer's signed up to new deals.
The probe centres will check if the telecoms giant failed to give people "clear and simple" contract information for new customers.
Since June 2017, telecom providers have been required to give customers contact information and a short, usually one page, summary of the main contract terms before signing up.
The summary must include key information about the price, length of the contract and the terms and conditions if a customer decides to end their contract early.
However, an investigation has been launched as Ofcom has "reasons to suspect" Plusnet, which is a BT subsidiary, may have "failed to comply with these requirements".
This comes after the regulator opened an investigation into EE, another BT subsidiary, in October 2022.
As a result, the Ofcom investigation will now consider if BT has breached Ofcom rules because of the suspected breaches at both EE and Plusnet.
The investigation into EE has now closed but Ofcom will be merging this investigation with the one into BT.
BT could also be fined by Ofcom and asked to change its practices.
Ofcom said it will continue to gather further information and will provide updates for customers as its investigation progresses.
Alongside the contract summary, Ofcom said customers with disabilities can request to receive the documents in an accessible format free of charge.
For example, suppliers must provide the same information in large print or Braille.
Ofcom says these rules are intended to help all customers make "informed choices" about the services they are being offered.
Commenting on the announcement, Rocio Concha, director of policy and advocacy at Which? said: "It's extremely concerning that BT is under investigation for potentially failing to provide clear and simple information for new customers at a time when customers under contract with EE, Plusnet and BT are facing huge above-inflation hikes to their bills.
"With the cost of living crisis continuing to bite, Which? is calling on providers to let their customers leave their contracts without incurring exit fees to switch or negotiate a better deal if they face price hikes - regardless of whether or not these increases can be said to be ‘transparent’.
“Providers should also carefully consider the level of any mid-contract price hikes when many consumers are already under huge financial pressure.”
A BT spokesperson said: “We want our customers to be fully informed and we make sales information upfront, clear and transparent. We are fully engaged with Ofcom during the course of this investigation.”
