Quick reply...
Is it a possibility to send SMS through your own SMS centre bypassing normal service providers?
Conversations->Settings->SMS centre number
Maybe, you could set up laptop at home to receive SMS as SMS centre, and to send them through your PC Internet connection for free (you know, websites which allow you to send SMS for free). Then set SMS centre number in your N900 to laptop's number, and send SMS through Internet for free.
Or, if you are good at compiling, compile SMS centre at your N900 and set its number as SMS centre. Then N900 would use its own SMS centre and try to send SMS directly to any nearby (several kilometers) phone.
I just don't know. It's the strangest setting I have ever seen.