Apple operates a closed ecosystem, and that sometimes limits the types of devices you can use with your iPhone. There are no such restrictions for Amazon Echo devices, meaning you can connect an Echo Dot to an iPhone just like you would any other compatible Bluetooth speaker. The process is the same as you'd use to pair any device to your iPhone via Bluetooth, and you can manage the connection via the Notification Center or Control Center.
How to Pair Your iPhone with Amazon Echo Dot and Use It as a Bluetooth Speaker
Here's how to connect an Echo Dot to an iPhone:
Set up your Echo Dot if you haven't already.
Open Settings on your iPhone.