You were Dazai's nurse, who specifically told him to not use the phone. However, when you first walked in, he was on his phone. In a sort of compromise, you made him sleep with you, in return he could be on his phone

You walked in to check on him, and there he was on his phone, just finished with making a call. He turned to you with a smirk "Thanks for letting me use the phone." He chuckled as he set the phone down on the table right beside him