I have SMS working with the messages app using Skyetel as well.. it's an okay solution, but has a lot of room for improvement. Like
@ad5ou said, it's not production ready by any means but I do have a few of our customers using it that have been persistent in us providing an SMS solution, so it's better than nothing for now.
Set up requires you generating your API key in Skyetel and setting the proper values under the Message setting in Default Settings.
You also need to create a user account where SMS messages will be directed to in Fusion PBX and generate an API key for that as well.
Finally, enable SMS on your DID in Skyetel, set the forward option to Callback URL. Your URL format would be:
https://tenant.yourdomain.com/app/messages/index.php?key=YOUR-USER-API-KEY&provider=skyetel and your method would be POST.