Would it be possible to turn the field test into an app?
Field Test is already an app: /Applications/FieldTest.app!

The app icon is just hidden on the home screen, and the Phone app launches this app.

If your iPhone is jailbroken, you can edit the application Info.plist to show the icon on the home screen. In /Applications/FieldTest.app/Info.plist, edit SBAppTabs array to remove ‘hidden’, then run uicache as mobile.