My iPhone has been hacked as l am not able to receive and make calls over the last three days.I have shut I completely and re-open it to make a call.

Check your iPhone settings

Go to Settings and turn on Airplane Mode, wait five seconds, then turn it off. Check Do Not Disturb. Go to Settings > Focus > Do Not Disturb and make sure it's turned off. Check for any blocked phone numbers.

If you can't make or receive calls on your iPhone - Apple Support

By hack, if you mean a stranger is remotely controlling or monitoring your device, then...

Keep the iPhone updated to the latest iOS always and never Jailbreak. That's it.

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware if it is updated to the latest iOS/iPadOS unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed them on your device or/and have Jailbroken. 

It (Hacking) also depends on how careful are you in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password, etc with your friends and family members.

Be careful when sharing the device's sensitive and valuable information with friends and family members.

Thumb Rules:

1. Enable  Guided Access before handing over an iPhone to kids or to a stranger.
2. Don't share Apple IDs
3. Don't Jailbreak
4. Don't share sensitive information pertaining to your device and payment methods
5. Beware of Phishing
6. Do not share your location with a stranger --> Find friends and share your location with Find My
7. Do not leave your iPhone unlocked and unattended in public places like offices, schools, malls etc.
8. If you see this --> What to do if you get an alert that an AirTag, Find My network accessory or set of AirPods is with you