> What does a hang up look like? Does it actually say BYE?

Yep, in plain text even :-) And WireShark will decode it all nicely for
you. The whole SIP-protocol is specified here:

[url]http://tools.ietf.org/html/rfc3261[/url]

Some examples of what you might see come by can be found here:

[url]http://tools.ietf.org/html/rfc3261#section-24.2[/url]

If the call hangs up because of a reported error from the other side, you
will also see the (numerical) SIP-error-code on the display of your
S450IP-handsets, you don't even need wireshark for that :-)

Christian