"Save as known" saves in the app only, and is only used for spam control of embedded images.
If you wish to save the sender in the Contacts / People app, tap the sender's image (in the message header, top left) and you'll be prompted to do so.
If the sender already exists in Contacts / People, you'll be prompted to open the contact's data card, or -- did you know this? -- call him or send an SMS... provided there is a phone number saved in the card too.