I finally received a test text and I think i've figured out what was wrong with sending my other texts. The number you're sending to needs to have the country code in front of it.

For example, in UK the number should start "+44", then drop the first '0', then carry on with the rest of the number, e.g. if the number you want to send to is 077******** and you use that then it fails to send. It seems you'd need to use "+4477********" for this app to work.