Annoying myriads of Potential Spam calls

Avoid unwanted calls on iPhone

1. Tap Favorites, Recents, or Voicemail. Tap. next to the number or contact you want to block, scroll down, then tap Block this Caller.
2. Tap Contacts, tap the contact you want to block, scroll down, then tap Block this Caller.

Avoid unwanted calls on iPhone - Apple Support

Are you seeing something as shown in the pic below? This is neither a hack nor a virus. It is spam and you can get rid of it. Read below...

Then probably you have accepted an offer or subscribed for a spam Calendar accidentally. Watch the GIF below to know how to unsubscribe from these alerts. Look for an unknown Calendar on the device under subscribed calendars.