MacSparky, of MacPower Users fame, has a cool trick for being more productive when calling people who have an extension for their telephone number. Instead of entering the phone number in contacts as 866-6790×1567 use the special character (;) for prompting the extension 866-6790;1567 so when you call you will be prompted to add those 4 digits with one tap. You can also use the comma (,) for those cases where you want to automatically enter numbers after a short delay.
You can set the commas and semicolons in the Contacts app on your Mac, which is obvious. Not so obvious is the fact that you can add commas and semicolons on your iPhone too. To do so, press the symbol button on the dialer and then “pause” for a comma or “wait” for a semicolon.
You can use dashes (-) and spaces in the phone number to make them easier to view because these characters are ignored. To find the extensions in Contacts to convert to the ; character just do a search for the character x if that is what you used previously.