Whatever she thinks about the turning up at school business, I think she's absolutely certain he's controlling who she sees and is restricting what she does. The moment she was free to, after he'd told her about the taxi arrangement, she was on the phone to Kirsty like a parched woman in the desert seeing an oasis.
I think keeping the phone in the drawer is part of this: she knows he wants to control that too. But the thing is, if he finds her phone and looks through it and finds out there is a lock on it, there will be some spurious request to use the phone in Helen's presence, which she will be unable to say no to (Silly me, darling - I've left my phone at the shop. You don't mind if I use yours, do you?) Once he finds there's a lock on it, she will drown in a welter of passive-aggressive don't you trust me, darling? You really are thinking the silliest things etc etc etc. Far better to have a secret phone. Hopefully that's what she's keeping in the drawer.