The other party sends a 200 (OK) to accept the change.
The requestor responds to the 200 (OK) with an ACK. If the other
party does not accept the change, he sends an error response such as
488 (Not Acceptable Here), which also receives an ACK. However, the
failure of the re-INVITE does not cause the existing call to fail -
the session continues using the previously negotiated
characteristics. Full details on session modification are in Section
14.