BT is continuing to develop its MVNO offering with the launch of pay monthly contracts.
The UK-based operator, which spent €12.5 billion on acquiring EE, began offering a SIM-only mobile product last year.
According to its latest figures, BT Mobile has signed up 400,000 subscribers.
EE has begun life at BT as a standalone unit.
The new BT Mobile offering features three plans with a starter 400 minutes, unlimited texts and 500MB of data costing £20 a month.
Consumers can choose from devices including the iPhone 6s, Samsung Galaxy S7 and Sony Xperia X.
BT broadband customers get a £5 monthly discount.
The operator said the launch would be supported by a nationwide advertising campaign across TV, cinema, radio, digital and social media.
BT Consumer CEO John Petter said: “Just over a year ago we launched some of the UK’s best value SIM-only offers from just £5 a month.
“Now we’re going one step further with our fantastic value pay monthly mobile phone contracts with extra savings for loyal BT broadband customers.
“We’re offering great deals on great phones, 4G on the UK's biggest network with unlimited access to the most extensive Wi-Fi coverage via more than five million BT Wi-fi hotspots.”
The launch comes amid a period of uncertainty in the UK mobile market following the failure of the merger between O2 and Three.
The European Commission blocked the deal on competition grounds last month, with Three parent CK Hutchison threatening to mount a legal challenge.