reject caller with a message

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hey mates,
windows mobile has the option to reject a caller with a message (i hope i translated that right, in german "Anruf mit Nachricht ablehnen").
the problem is, that i sometimes take anther phone with me which has got another phonenumber. most of my friends don't know it and so i want to let them know, that i have got another phone with me..
but when the option is checked and someone calls me, i have to press on right softkey "Send SMS".
is there anyway or an app which sends it automatically? is it even possible to reject SMS with this?
thanks in advance,

DN41