how to dial this from ivr ?

Hello,
to call a certain germain telefone feature i need to call this from an ivr ...

[custom-cd]
exten => s,1,Answer()
exten => s,n,Noop(Enable Callforward)
exten => s,n,Set(_MISDN_KEYPAD=*21*04031819288*32512538#)
exten => s,n,Hangup

I added it extension_custom.conf and as custom destionation. But i get this error:

[Dec 15 19:47:57] VERBOSE[10041] logger.c: -- Goto (custom-cd,s,1)
[Dec 15 19:47:57] WARNING[10041] pbx.c: Channel 'mISDN/5-u12' sent into invalid extension 's' in context 'custom-cd', but no invalid handler

Can you help me, where i think wrong.

Thanks and best regards

Niels