First check with your provider what the rates are for international calls.

Second, make sure you know the country code of the international number you are trying to call. Now that you are sure of these two important things, you can start calling.

Launch the Phone app on your iPhone. International calls start with +. Press and hold the 0 key until the + appears. Enter the country code of the number you are calling and then enter the local number. For example, if you call someone in the UK, you would dial 0(+), 44 and then the local number.
Once the entire number is dialed, select the green call button to start the call. If you plan to call this international number again later, it's easier to save it as a contact.

Please take the opportunity to connect and share this video with your friends and family if you find it helpful.