I have not yet tries the Email2SMS API. To circle the issue down, you can do this:
1) You may add a CC address t your address entry, so you see if the email was actually sent. And to what address.
2) After logging in to LOX24, you shall see if the SMS was received for delivery.
When using www.soscisurvey.de or s2survey.net, we can offer sending SMS directly via LOX24. The costs are 0,10 € plus tax per SMS in this case, and we will account at least 200 SMS.
When using a local server (e.g., a campus server), you can configure a LOX24 account to use the SMS feature of SoSci Survey.