Try Calling With a Different App

If your regular Phone app is acting up, you can try using another app to call them. With so many apps on your iPhone, you’ll surely have an alternative you can use, and nearly all of these will work over Wi-Fi or your cellular data connection, bypassing your carrier’s voice network.

For instance, if the other person is an iPhone user, you can call them via FaceTime. Likewise, you can also use other apps like WhatsApp or Messenger. If they don’t pick up, that means they’re probably the ones who can’t answer the call right now.