Suspicious Calendar Events, Spam Phone Calls, and Outgoing Messages—Has my iPhone been hacked?
Most likely, your Apple ID was hacked. 

Any device signed in to your Apple ID can send texts, change notification sounds for specific contacts, change contacts phone numbers and more.

Go to appleid.apple.com to change your password, and change passwords on any sites that you use the same password on, as those are at risk as well.

If your password has been changed by the perpetrator, you will most likely have to contact Apple support, as well. 

To prevent this from happening in the future, I recommend adding two-step verification to your account, so every log in has to be approved via a code on one of your devices or phone numbers.