Reinhard, first I want to say THANKS to you and the Avast team for bringing such a comprehensive tool to mobile users for free. I have always believed that security is a right for all, and not a privilege.
Second, I believe what epiccelt is getting at is the same issue I'm having, on the same device. On CDMA networks (in our case Sprint in the US), unless the message is split on the sender side into two separate messages, the receiver, regardless of network or device, receives jumbled nonsense instead of a text. Many third-party SMS apps have the ability to do so seamlessly, and I don't think it would be too tough to add to AMS. I have verified this issue on multiple devices from different networks, and across the same network on similar devices; I have also done so with a network-agnostic service (Google Voice). I hope this can give you some insight into working on the issue. Other than that, the service works as advertised. I eagerly await the web interface as well. Please let me know if this helps.