Ovislink OV804WV User Manual page 191

Figure 154 Registration
Simple Call Flow
Sometimes SIP user agents know the exact location of each
other, and they are configured without proxy. In this case, both
can talk directly.
Alice seizes here phone, keys in Bob's number, in SIP URL
format. Assuming Bob is using a SIP-enabled IP phone with IP
address 100.100.100.1, Bob's SIP URL should be something
like . After Alice presses Dial button on her
bob@100.100.100.1
phone, a SIP INVITE message is sent to Bob's IP phone
directly. Once Bob's IP phone receives this message, it rings,
and meanwhile replies with another SIP message to Alice, and
then Alice hears a ring-back tone.
Bob knows a incoming call is available, and off-hook his phone.
At this time a 2-way voice connection is created, and both
parties are able to hear and talk with each other.
In this example, Bob first on-hooks his phone, producing an
ACK message sent back to Alice. The arrival of this message
terminates the voice connection, making Alice hear a busy tone
on her side.
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