I think the reason she attacked star when she argued with frankie or they split up is because SB was pissed that frankie was always choosing Star over her and it made SB furious and jealous. e.g the "I just want Star" and other texts Frankie sent when SB was refusing to bring her back etc. She thought if she got rid of star frankies attention will be 100% focused on her , all to herself .
I think she could have brought the kids with her so that she could go home and get rid of her phone after possibly so she could stage the scene and if anyone questioned what she was doing taking so long to get to the hospital she can say she had to drop the kids off. She didn't have her phone at the hospital when police asked for it . Becky endacott had to bring SBs phone to her sisters house where they were staying when police came round to see them again a few days later I think it was. She attacked star, went to drop off the kids at beckys and left her phone there, knowing police would probably ask for it. So her phone call whilst she was the car driving was most likely before she dropped the phone and kids off at beckys
Im sure it was mentioned that those 11 minutes taken to call an ambulance SB could have been telling FS what to say to everyone and what the story needs to be, so how does the call to her mum mean she must have been telling the truth, if that call was after ? Those 11 minutes, and the time they were in the hospital room together , that's two instances