Certain audio types such as for phone calls or VOIP services are called "Telephony" services. Apple has built the iOS operating system with certain restrictions that only allow the built-in microphone or bluetooth to do audio for these Telephony protocols. If you are using an app (like Skype or Facetime) that uses Telephony rather than regular audio, then you can't use your Apogee interface with that app.
This is different than regular audio that you'd use for recording into Garageband or Music Memos, streaming apps like Facebook Live & Zoom, or even audio for video such as the Camara app in video mode. You can use your Apogee interface with those iOS apps because they use regular audio protocols.
Note: On a Mac or PC computer, these restrictions do not exist. So you can use your Apogee interface with Facetime or Skype on your Mac or PC computer with no problem.