On the last day of the plot of Before I Go to Sleep, Ben/Mike is shown in his car as he leaves the house for the day. On the passenger seat is a second phone that is ringing. That phone shows that the call is from Claire.
We know from a prior conversation between Christine and Claire that Christine had never revealed Mike's name to Claire. We also know that after Christine's description of Ben/Mike to Claire, Claire states that she does not know who Christine is living with.
How can Claire be calling Ben/Mike if she knows the real Ben but does not know the fake Ben?