Разговорный тракт в IP проключается, в основном, по ответу. А здесь ответа нет. Цитата из доки:
Figure 6.7: Example of a dialog
The exchange shown in figure: Example of a dialog includes 2 transactions.
The first transaction begins with the INVITE request fr om Alice to Juliette and ends with a non
1xx response; in the example, the OK response from Juliette:


1. Alice sends an INVITE request to her proxy server for a call to Juliette. This request
contains an SDP description of the media that Alice wishes to use,
2. The proxy server determines Juliette's proxy server address, for example by consulting a
DNS server, transmits an INVITE request to this server and a 100 Trying response to
Alice,
3. The second proxy server transmits a 100 Trying response to the first server and consults
its location server to find Juliette's actual address. Once this address is identified, the
INVITE request is sent to Juliette's SIP terminal,
4. Juliette is informed of the call when her terminal rings and a 180 Ringing response is sent
to Alice's terminal. This response contains, in the Contact field, Juliette's current address
(wh ere she can be contacted directly without transiting via the proxy server),
5. When Juliette off-hooks, a 200 OK response is sent to Alice's terminal. This response ends
the transaction. It can contain an SDP description of the media that Juliet wants to use in
relation to Alice's suggestion,
6. The second transaction begins with Alice's acknowledgement ACK. The ACK request is
transmitted to Juliette's URL, contained in the 200 OK contact field.
RTP/RTCP voice channels on IP are established between the two terminals, in compliance
with the results of SDP negotiation,

7. Two messages (BYE and 200 OK) end the dialog. RTP/RTCP channels are also released.