Steven's 'alibi'?

I could be wrong, but I believe he was trying to set up an alibi for his phone call. The one that brought her out into the open, made her vulnerable. He placed a call on his personal phone to his office. That call lasted approximately 10 or 15 minutes, spanning the time that the house phone rang. Then he used a second phone, presumably a burner -- and untraceable, to call the house at the pre-appointed time. Consequently, he would not be suspected as part of a murder plan that hinged on a phone call. Does that help?

PS ? I found all three lead characters quite unlikable and would've been happier if they all ended up dead.