I have a fistext account which I use with the fishtext connector for websms. Works great. If you don't know what these are, google the words. Sending messages is not free but very cheap, especially if you send them abroad.
What is your own SIP provider? Do they provide SMS? Diamondcard provides good voice and text (SMS) services.. their link here
Here's a link to my own copy of the app, but you can also send via the diamondcard.us site from your PC browser or the diamondcard lite admin centre via mobile browser