When creating a contact, instead of this:
The semicolon is secret code to your iPhone to wait once the call connects and gives you the option to dial the extension when you tap it at the bottom of the screen.
Alternatively you can use commas to have the phone delay slightly and enter a digit for you. For instance, if you frequently have to call your cable company to reset your cable box and you know the tone sequence to make that happen, you could have a phone number like this.
Cable Box Reset