The iPhone’saccessibility optionslet you set this as a default.
This option is yet another accessibility feature that benefits everyone.
How to Set Speakerphone Mode to Automatically Activate on iPhone Calls| OS X Daily
Set Your iPhone to Use Speakerphone by Default
If you're on a long drive or just prefer to use speakerphone, it's a pain to set it every time you pick up a call. The iPhone's accessibility option
