<p>I think this is covered by SIP RFC - a 423 reply with Min-Expires header has to be sent back.</p>

<ul class="task-list">
<li><a href="https://tools.ietf.org/html/rfc3261#section-20.23">https://tools.ietf.org/html/rfc3261#section-20.23</a></li>
</ul>

<p>I am not sure sending back 200ok with higher expires is RFC compliant, but if it helps with some user agents, it can be made optional.</p>

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