Caller              OpenSIPS                 Core IMS (domain)
    |                     |                        |
    | REGISTER alice@domain                        |
    | From: alice@domain  |                        |
    | To: bob@domain      |                        |
    |-------------------->|                        |
    |                     |REGISTER alice@domain   |
    |                     |From: alice@domain      |
    |                     |To: alice@domain        |
    |			  |(Add some P-Headers)    |
    |                     |----------------------->|
    |                     |                        |
    |                     |  401 auth required     |
    |                     |<---------------------- |
    |                     |                        |
    |                     |                        |
    |                     | REGISTER w/ auth       |
    |                     |----------------------->|
    |                     |                        |
    |                     |  200 OK (Headers)      |
    |  200 OK             |<---------------------- |
    |<--------------------|                        |
    |(Relay same headers) |                        |