1. I also have used Sparruf callback to my airBBalticcard several times personally. With SMS callback and dialed callback.
2. If you need me to register you, please drop me your data via PM. I'll call them for you.

, BUT:
You can also register directly on their website via www. Go to the GERMAN version of their site and you'll find a field "Jetzt anmelden" (means "register now"):
http://sparruf.de/anmelden.html
You need to enter your name, (christian name (=Vorname) and surname (=Nachname)), your telephone number (Ihre Nummer) and your email address. That's all.