Unfortunately, it is not possible to listen to music through the iPhone's speaker while you are on a call. This is because the iPhone is designed to automatically route all audio output to the call audio when you are on a call. This means that any other audio sources, such as music, will be muted until the call is ended.
However, there are some workarounds that you can try.
- One option is to use another Apple Device to receive calls such as a Mac or another iPhone.
- I personally would not recommend the option to use a third-party app that can allow you to listen to music while on a call, because these apps may not be reliable and your privacy may get compromised as well. All apps may not work well with all iPhones.
Overall, it is recommended that you focus on the call while you are on it and avoid multitasking with music or other distractions.