By default when register message passes SBC username in contact field is replaced with random string. This string identifies pinhole opened for particular user for management purposes. But this behavior is not always desirable. Many SIP implementations identifies user by Contact field and rewriting it with random string would prevent application from registration.
SBC can be configured to pass username set in Contact field. To do this you have to configure

    registration contact username passthrough

This have to be set on adjacency that should not rewrite username when SIP REGISTER comes from