A babysitter gets prank calls from a mysterious caller gets in a home invasion happens to be an child abductor who abused her in her childhood.

Here is a rough example:

"When she discovers her mysterious prank caller is the same man who abused her as a child, a babysitter takes matters into her own hands to stop him before he abuses again."

She wants to save a new victim.

Better! However, I would at least hint at a goal in the logline.

When she discovers the prank caller is the man who abused her as a child she would either want to get even/revenge or expose him. Maybe she just wants to destroy his life, Or perhaps he is abusing someone else and the lead wants to save the new victim.

Anyway, it's an interesting concept!