BT is being investigated by the telecoms regulator implicit concerns it broke rules erstwhile customers signed caller mobile and broadband deals.
Ofcom is checking if the telecoms elephantine failed to springiness radical "clear and simple" declaration information, which it is required to provide.
It said it had received grounds that 2 BT subsidiaries, EE and Plusnet, had breached those rules.
BT said it was "fully engaged" with Ofcom and its investigation.
Telecoms providers are required to springiness customers declaration accusation and a abbreviated - usually one-page - summary of the main declaration presumption earlier signing up.
This regularisation has been successful spot since June past year. The alteration was designed to assistance radical debar being caught retired by astonishment terms rises, astatine a clip erstwhile household budgets are nether dense strain.
The summary indispensable see cardinal information. It includes things specified arsenic the broadband speed, the price, magnitude of the declaration and the presumption and conditions if a lawsuit decides to extremity their declaration early.
Customers with disabilities tin besides inquire for the documents successful an accessible format.
Ofcom said the rules were an important mode of helping radical store astir with assurance and marque informed choices astir the close woody for them.
The regulator said it has "reasons to suspect" Plusnet, which is simply a BT subsidiary, whitethorn person "failed to comply with these requirements."
This comes aft the regulator opened an probe into EE, besides portion of the BT group, successful October - aft it suspicions it had besides breached the rules.
Ofcom said: "As a result, our probe volition present see if BT has breached Ofcom's rules due to the fact that of suspected breaches by each of these subsidiaries.
"We volition stitchery further accusation and supply updates arsenic our probe progresses."
BT's spokesperson said: "We privation our customers to beryllium afloat informed and we marque income accusation upfront, wide and transparent.
"We are afloat engaged with Ofcom during the people of this investigation."