Some SIP user-agents use INVITE without SDP offer to refresh session.
By default, NUA sends an offer in 200 OK to such an INVITE and expects
an answer back in ACK. Now nua tries to recover from such a Offer/Answer
protocol error.

Also, if NUTAG_REFRESH_WITHOUT_SDP(1) tag is used, and if the re-INVITE was
received without SDP, no SDP offer is sent in 200 OK.

Thanks for Anthony Minessale for reporting the problem.

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