Short Answer 
To set call audio routing on an iPhone, open the Settings app, navigate to Accessibility, and select the Audio/Visual option under Hearing. Then, choose the Call Audio Routing setting and select Speaker to ensure audio from calls plays through the speaker.
Step 1: Access Settings
Start by opening the Settings app on your iPhone. This app typically has a gray icon with gears, and it’s where all your phone’s features are managed. Once you have the Settings app open, look for the option to customize various functions on your device.
Step 2: Navigate to Accessibility
Scroll down within the Settings menu until you find the Accessibility option. Tap on it to access various hearing and vision features, which help enhance the user experience for those with different needs. This section houses the settings that will allow you to change how audio is routed during calls.
Step 3: Set Call Audio Routing
Under the Hearing section in Accessibility, select Audio/Visual. Here, you will find the Call Audio Routing option. Tap on it and choose Speaker from the list. This setting ensures that audio from calls will always play through your speaker, allowing you to enjoy music during calls.