> <-------------------UPDATE--------------------
>
>       (Session-Expires:3600;refresher=none)

The header is malformed.  The valid refresher values are indicated within
RFC 4028 section 4.


> [Query]: What should be the value of session refresher in 200 OK of
INVITE?

The refresher should be "uac".  RFC 4028 section 9: "as the table
indicates, the UAS cannot override the UAC's choice of refresher".

As indicated within RFC 4028 section 7.4, it is okay for the refresher
roles to change.  Within your example, the caller originally preferred to
be refresher; then it preferred called to be refresher; then the RFC
required the called to switch refresher back to the caller's original
preference.
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