Is my phone hacked?
Phones themselves are rarely hacked. Usually it's an account, a calendar subscription, or an app, and those can be checked.
What's usually going on
Most odd phone behavior has an ordinary cause. Spam calendar invites, a website that keeps sending notifications, an app with too many permissions, or an email account someone else has signed in to all look like a hacked phone. A genuinely compromised phone is uncommon, and is usually a targeted attack on a specific person.
Worth trying first
Update the phone.
Install the latest iOS or Android version in Settings. Updates close the security holes that real attacks rely on.
Check for profiles and admin apps.
On iPhone: Settings, General, VPN & Device Management. On Android: search Settings for "device admin apps". Remove anything you don't recognize that isn't from your employer.
Check your Apple or Google account.
Look at the list of devices signed in to your account and remove any you don't know. Then change the password.
Clear spam calendars and notifications.
Delete calendar subscriptions you didn't add, and turn off notifications for websites in your browser's settings.
Use the built-in checks.
On iPhone, Safety Check in Settings, Privacy & Security reviews who has access. On Android, run Google Play Protect from the Play Store.
When to hand it over
If you think someone close to you has had access to the phone, if you're a public person who may be targeted, or if the checks above don't explain what you're seeing. We'll go through the phone and every account on it with you, quietly and properly.
Your own technologist, on call.
Tell us a little about your setup. We'll take it from there.