the phone number format stored in your contact list can affect your ability to call a contact on your iPhone. Different countries have different phone number formats, and if the phone number in your contact list is not in the correct format, it may cause issues when you try to call that contact.
The correct phone number format generally includes the country code, area code (if applicable), and the local number. For example, if you are in the United States and the contact's number is (555) 123-4567, the correct format would be "+1 555-123-4567" (where "+1" is the country code for the United States).
If the phone number in your contact list is not in the correct format, your iPhone may not be able to recognize it as a valid phone number and could prevent you from making calls to that contact. In some cases, it may also lead to other issues like incorrect caller ID display or difficulties with messaging.
Spaces, Hyphen or Parentheses in a Phone number depends on the International prefix
Watch the GIF below. Note here only numbers are being input, but Spaces, Hyphen or Parentheses are added by iPhone automatically
iPhone 6 iOS 12

If you need to edit a contact's phone number to add or modify the format, you can tap "Edit" in the contact details and make the necessary changes to the phone number field.
By ensuring that your contacts have their phone numbers in the correct format, you should be able to call them without any issues.