Not quite "free for use". But, there are companies such as Twilio which specialize in providing APIs for sending SMS messages, so that individual web apps won't need to make agreements with dozens of telephone operators anymore.
Such services will usually route all messages (even from different customers) sent towards the same regio through the same phone number. For example, I live in Central Europe and various services – Tumblr, Blizzard, eBay, Namecheap – send me security codes via +44.7781470659. (According to the web, even Google and Microsoft use the same gateway.)