If you paste or otherwise copy a phone number containing letters (example: 1-800-FLOWERS) into the iPhone’s Contacts app, iOS can figure it out and dial it for you.
Editor’s note: We were specific with “paste” because as you can tell, when entering a phone number into Contacts, iOS displays only a numerical keyboard, not the full alphabetical keyboard. But you can paste numbers into that field or get them in there by tap-holding on them in emails and webpages.