I think what you'll want to do is use a recording proxy to watch the actual data flowing with GET/PUT/POST and duplicate that. You should also check that you're following the Terms of Service; if you're having your server ping you with a message when it goes down, you should be fine.

If you're sending commercial messages, note that that's not permitted under the TOS:

...Way2sms.com is a free messaging site for person to person communication and is not meant for commercial communication like marketing, promotional or advertisement messages. Such commercial messages are Restricted Messages. If a Registered User is found to send Restricted Messages, his account will be completely blocked.

So automating messages seems to be legit, but using automated messages for spam will get you shut down. It's up to you to determine if you're sending legitimate messages or not.