This is typical of the kind of project questions we get here from electronics students who want to pass their course but at the same time don't really care to take that much interest in the electronics subject itself. Most of us forum users quite like helping people with their projects and giving general electronics advice, but we generally like to see that some effort has been undertaken by the project author first. Afterall, who wants to do someone elses entire project for them?
It sounds to me like you've still got a blank piece of paper at the moment. You really need to do some research into project viability for the various ideas that you have. It's ok to be ambitous, but you need to ensure that you're not exceeding you're own capabilities. At the end of the day all you really need is a BTEC standard project - as long as you fulfill the needs of the syllabus for a BTEC standard project you're ok. Don't go digging yourself into a deep hole for the sake of it, especially if you find that you can't get yourself out of that hole. It's too late three months down the line to decide you can't do your project and that you want to change your idea - so be very careful when choosing one!
With regards to SMS projects, my initial thoughts are that this kind of project is too complex for a standard BTEC project. However, it's up to you to decide whether it's a viable project or not and the best way to do that is to evaluate the work that previously went into a similar project.
If you search the archives of Elektor Electronics Magazine (visit their website) you should find that in one of their back issues, going back some time ago now, they did a whole project on control over SMS. Dig this article out, you'll probably have to pay a couple of pounds for it, have a read through and then, once you've an understanding of what's involved you should be able to decide whether you think you can do an SMS related project. The article will also give you a good grounding on the subject should you wish to continue in this direction.
Let us know how you get on.
Brian