Great question. There are all kinds of hidden call recorder apps out there that claim to let you listen to cell phone calls remotely. Whatever you choose, make sure it also includes functionality that lets you review who they called, when they called them, and how long they spoke for. Remember, knowing all the details will help you better understand the types of conversations they’re having. If they’re calling the same number repeatedly, and speaking for a long time, it’ll help you confirm your suspicions.