I've looked and looked for a SMS service which is both reasonably priced, cheap, and easy to use. Clockwork is all three of these. Their prices are some of the best I've seen, their API to incorporate SMS services into existing or new projects (in almost any language) are great and easy to follow with some handy and easy to read documentation, and finally their user interface is simple yet powerful. I've found a lot of SMS services confine you a single country, locking you down to local number and forcing you to pay extra to send to other countries with both set-up fees, more SMS costs and actually paying for a number. Whilst this might appeal to huge companies focused in one country who wish to send from their own private number - Clockwork has not got these limitations. Whilst different countries have different SMS costs, I am able to send to any number anywhere with reasonable rates. This has made my life as a developer easy, a walk in the park if you will when it comes to "Two Step Authentication". As mentioned before, I have probably demoed about 30 similar texting services, and whilst they all have their quirks, good things and bad things. There is a solid reason I have chosen Clockwork, and I don't intent to move.