Error sending message

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Hey!!
Hope u can help me! I've got a big problem using OpenNETCF.Phone.

private void Senden_Click(object sender, EventArgs e)
{
SmsAddress sa = new SmsAddress("+436803041981");
OpenNETCF.Phone.Sms.Sms d = new OpenNETCF.Phone.Sms.Sms();
d.SendMessage(sa, firstKenn);
}

Now this works perfectly on Window Mobile 2003... But on using Windows
Mobile 05 i get the error: "Error sending message"!

Hope u can help me, i've got no idea...
THANK YOU
.