Asterisk TO header and DNID

Нарыл на одном форуме, так как надо было передать номер набраного DID клиенту.

The Asterisk dial string can provide a DNID to override the To header; you place it after an "!" at the end of the dial string. However, this is only of any use if the phone knows what to do with it. Also a phone probably won't make a distinction between the invite URL and the To header one, so simply providing DNID as the digits to dial will probably work.

Dial(SIP/section/digits!DNID)

Оно работает, но почему нигде не документировано?