By default, when you receive a FaceTime video call on your iPhone, the speakerphone kicks in immediately after answering. However, it's the exact opposite when it comes to FaceTime audio calls. Luckily, this is pretty easy to remedy if you'd rather have the speakerphone kick in instead of the built-in ear speaker.
Before we being, a few things to note. First, if you're connected to wired headphones or a Bluetooth set, no matter what option is set, all audio will go through those devices. Second, the instructions below will also make speakerphone the default for regular phones calls. You can't tweak this setting for Phone or FaceTime individually.
Now that that's out of the way, head to the "General" section in your iPhone's Settings app, then tap on "Accessibility."
Next, choose "Call Audio Routing" from the list, and you'll see that "Automatic" is already checked unless you changed it before. Tap on "Speaker" to check mark it as the default. With this option set, whenever you're not using wired headphones or a Bluetooth speaker, the speakerphone will automatically be enabled when making and receiving FaceTime audio calls (along with regular calls in the Phone app).
This process will work on any iPhones running iOS 8 or higher. In older iOS versions, the steps were slightly different.
Now, whenever you make or receive a FaceTime audio call, the "Speaker" icon will be automatically toggled on, so you don't have to worry about it. If you want to use your ear speaker instead, just tap on the icon to disable it during a call.