Hello.
First of all, even though i have no experience with SMSsync (see how smoothly I've integrated the "am I robot?" control word here :)), I have experience working on Ruby on Rails powered back ends. Among others, I worked on an Estonian (.ee) top level domain maintenance and registration system which itself was a rather complex web service with both restful (JSON) and old school XML APIs.
I went through the provided documentation (and example) and I should be able to convert this to Ruby. Although, I have a question about this, isn't rails a bit of an overkill for such an app? Ruby on Rails has a huge overhead and some of it could be cropped but maybe Sinatra would be more suited for such a project?
Not disputing your design decisions though and if it's needed in Rails then that's what I'll do.
Why did I bid so much less compared to some other bids? Well, by your provided description, which I have to say was very detailed and I'm thankful for that, a lot of people here don't do this, I can give a pretty good estimate on the workload and I don't see enough work here for 700-800$.
Looking forward to hearing from you.
Best wishes,
Andres K