Hi,
Please check following excerpt from RFC 5621;
Legacy SIP UAs without support for 'multipart' bodies generate a 415
(Unsupported Media Type) response when they receive a 'multipart'
body in a request. A UAC sending a 'multipart' body can receive such
an error response when communicating with a legacy SIP UA that
predates this specification.
It has been observed in the field that a number of legacy SIP UAs
without support for 'multipart' bodies simply ignored those bodies
when they were received. These UAs did not return any error
response. Unsurprisingly, SIP UAs not being able to report this
type of error have caused serious interoperability problems in the
past.
So it can be perceived that if UA can not interpret multi-part message body, it can either reject the request or ignore the content and proceed accordingly.
IMO, SIP implementors group will be better place to put this question and get some more feedback's as well.
- Vivek