Open the Phone app
Dial the main phone number
After the call connects, tap the keypad button
Enter the extension number
If needed, add a pause by holding the asterisk key until a comma appears
If needed, add a wait by holding the pound key until a semicolon appears
Save the number with the extension in Contacts for future use
To save it, enter the number, then add the extension after a comma or semicolon
Tap Call to place the call with the extension