The VIP version of Cardboard Fish allows you to save your contacts in your personal Phonebook.... but it costs money to sign-up for the VIP version (although there are also other advantages too, such as no advertising, receiving replies, etc...).
Depends how much you want those features as to whether it's worthwhile paying their charges or just going with the freebie version. According to their site, here's how much it costs:
VIP Account Signup
FREE - There is no setup or connection fee at the current time.
VIP SMS Credits
£0.05 - Each credit is five pence sterling. One credit = one SMS.
Personally, I just use the free ones cause it's not worth it for the amount I send.

- have you seen their free ringtones too? Listed in the banner at the top of the page. I showed this to a few friends that used to pay several pounds for a ringtone, and they were delighted.