One of the advantages of Linux is that it's free, i.e. you can choose what application to use at no further cost. That makes it easy to try all kinds of those applications to find the one you're most happy/comfortable with. Yes, that requires so homework on your part, but as you're the one using it, that's kinda mandatory, isn't it?
One of the links on the Google page is kannel. It so happens it has Debian packages, be it a slightly older version (1.3 vs 1.4 current). That would be the easiest option. As for commercial deployment, that's explicitly prohibited by most (all?) of these programs, so you'd be looking at obtaining a licence.