SIP 404 handshake

I’m doing interoperability testing with my service provider. When generating a SIP 404, Asterisk is supposed to precede the error with with SIP 100. But it doesn’t do that. Does this imply some configuration value I’m missing?

I generate the error by simply removing the inbound extension from my dial plan. I believe this is the conventional way to generate this error.

Thanks for any help on this. I’m really stuck.