407 Proxy Authentication Required problems

I've configured auth-attributes on ITSP SA:

 

auth-attributes
                   auth-realm example.com
                   username usernamehere
                   password ********
                   in-dialog-methods INVITE

 

I receive 407 Proxy Authentication Required from ITSP but the SBC just sends it back to the PBX instead of answering with a new INVITE with the credentials.

 

What am I missing?

 

Thanks!