To neutralize the threat of Misa being tracked or revealing too much, Light manages to take one of her three cellphones, establishing a strict protocol where only he can initiate contact.
L publicly declares to the Task Force that if he dies within the next few days, Light Yagami is definitively Kira . This bold move effectively prevents Light from using Misa's eyes to kill him immediately. The Downfall: Misa’s Arrest Death Note Episode 15
The episode's climax centers on the physical evidence L has meticulously gathered: To neutralize the threat of Misa being tracked
Light finds himself in a corner when Misa arrives at his school, To-Oh University, unexpectedly. Death Note Episode 15