If you're on a long drive or just prefer to use speakerphone generally, it's a pain to set it every time you pick up a call. The iPhone's accessibility options let you set this as a default.
This option is yet another accessibility feature that benefits everyone. To set your iPhone always to pick up on speaker:
- Go to Settings > General Accessibility;
- Tap Call Audio Routing in the Interaction section;
- Change the setting from Automatic to Speaker.
Now your iPhone will always pick up in speaker mode. You'll still be able to switch back to handset or Bluetooth, but it will default to speakerphone first.