Could you implement a feature to send an SMS via a web form from "My giffgaff" or similar please? Some other mobile carriers offer this.
It's just more convenient to send a text with a full keyboard when I'm near a PC. It would be particularly handy if you could stay logged in on a cookie, so you could keep the web form bookmarked without having to type a password. It would also need a phone book feature for frequently used numbers to be useful.
Would be even better if it was a fraction cheaper since you don't have to accept the message from the network :-)