Question:

An SMS firewall product needs to be able to intercept incoming SMS messages. What is the lowest level at which we can observe/handle incoming SMS? Is there a way to observe incoming SMS messages (other than CMsvSession)?

Answer:

It is not possible to intercept all incoming SMS messages before they reach the SMS mesaging

While it is possible to register to get notified of all messages, this option is only available to one client on the phone. A firewall application will be unable to register for this notification because the native messaging application registers too early in the boot sequence. Even if the firewall application was able to receive notification of the new message, it would be in a race with the messaging application to get the message - there is no guarantee it would be successful.

See also:
• Symbian FAQ: FAQ-1576: How do I intercept SMS datagrams based on the port number?
• Symbian Developer Network: Easy API for sending and receiving SMS datagrams