We're using email-to-SMS to send reminders. For T-Mobile in the Netherlands, we're sending reminders to phonenumber@gin.nl -- do you know if this email address format reaches your phone? For some mobile providers, it's necessary to enable the email-to-SMS service with them first before the reminders will work -- unfortunately I'm not sure how T-Mobile there works, but someone else on the forums might.

Thanks for the reply. I found out you (indeed) have to activate this service (for T-Mobile send 'EMAIL ON' to 888). Unfortunately it will cost you 0,30 cents per received message.

I'm using Google Calendar too (with free SMS notifications, (I know Google pays for these)), so I would really like to see the possibility of having RTM tasks with a due date/time to be added automatically to Google Calendar so I can receive free SMS notifications for these.