It is in the settings of the N900: menu > settings > device lock. To all, review the settings of your SMSCON with the Editor and activate the one you need and if you care to avoid costs of SMSs that you don't want to be sent. e.g. in the config proposed in the wiki page my preferences are more relaxed that the ultimate notification proposed by default. In short, profit from the effort made to make all those adjustable settings and not fixed in the daemon by tuning it to your preferences. cheers