"direct" SMS sending?

[rhn]

Hi
I've came up with an idea which might be completely useless or very convenient, but I don't know enough about GSM to be able to judge it properly.
As far as I know, the SMS messages are sent to a "message center" number (which can be edited by the user on some devices) provided by the operator using some lossless data transfer method.
Would it be possible pass the number of another device and send messages directly?
I suspect it wouldn't be possible to use an unmodified device to receive messages sent that way.

The whole idea was based on the fact that a voice connection over the time of sending a message costs less than sending a message to a different operator (at least around here). I don't know how would they charge for that kind of connections, though.

Does this even make sense?

rhn