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<h2><a href="https://wiki.asterisk.org/wiki/display/AST/Pre-Dial+Handlers">Pre-Dial Handlers</a></h2>
<h4>Page <b>added</b> by <a href="https://wiki.asterisk.org/wiki/display/~mjordan">Matt Jordan</a>
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<h2><a name="Pre-DialHandlers-Overview"></a>Overview</h2>
<p>Pre-dial handlers allow you to execute a dialplan subroutine on a channel before a call is placed but after the application performing a dial action is invoked. This means that the handlers are executed after the creation of the caller/callee channels, but before any actions have been taken to actually dial the callee channels. You can execute a dialplan subroutine on the caller channel and on each callee channel dialled.</p>
<p>There are two ways in which a pre-dial handler can be invoked:</p>
<ul>
<li>The '<b>B</b>' option in an application executes a dialplan subroutine on the caller channel before any callee channels are created.</li>
<li>The '<b>b</b>' option in an application executes a dialplan subroutine on each callee channel after it is created but before the call is placed to the end-device.</li>
</ul>
<p>Pre-dial handlers are supported in the <a href="/wiki/display/AST/Asterisk+11+Application_Dial" title="Asterisk 11 Application_Dial">Dial</a> application and the <a href="/wiki/display/AST/Asterisk+11+Application_FollowMe" title="Asterisk 11 Application_FollowMe">FollowMe</a> application.</p>
<div class='panelMacro'><table class='warningMacro'><colgroup><col width='24'><col></colgroup><tr><td valign='top'><img src="/wiki/images/icons/emoticons/forbidden.gif" width="16" height="16" align="absmiddle" alt="" border="0"></td><td><b>WARNINGS</b><br /><ul>
<li>As pre-dial handlers are subroutines, they must be terminated with a call to <a href="/wiki/display/AST/Asterisk+11+Application_Return" title="Asterisk 11 Application_Return">Return</a>.</li>
<li>Taking actions in pre-dial handlers that would put the caller/callee channels into other applications will result in undefined behaviour. Pre-dial handlers should be short routines that do not impact the state that the dialling application assumes the channel will be in.</li>
</ul>
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<h2><a name="Pre-DialHandlers-Syntax"></a>Syntax</h2>
<p>Handlers are invoked using the same nomenclature as other <a href="/wiki/display/AST/Asterisk+11+Application_Gosub" title="Asterisk 11 Application_Gosub">Gosub</a> routines.</p>
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<pre>b([[context^]exten^]priority[(arg1[^...][^argN])])
B([[context^]exten^]priority[(arg1[^...][^argN])])
</pre>
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<div class='panelMacro'><table class='infoMacro'><colgroup><col width='24'><col></colgroup><tr><td valign='top'><img src="/wiki/images/icons/emoticons/information.gif" width="16" height="16" align="absmiddle" alt="" border="0"></td><td>If context or exten are not supplied then the current values from the caller channel are used.</td></tr></table></div>
<h2><a name="Pre-DialHandlers-Examples"></a>Examples</h2>
<p>The examples illustrated below use the following channels:</p>
<ul>
<li><em>SIP/foo</em> is calls either <em>SIP/bar</em>, <em>SIP/baz</em>, or both</li>
<li><em>SIP/foo</em> is the caller</li>
<li><em>SIP/bar</em> is a callee</li>
<li><em>SIP/baz</em> is another callee</li>
</ul>
<h4><a name="Pre-DialHandlers-Example1Executingapredialhandleronthecallerchannel"></a>Example 1 - Executing a pre-dial handler on the caller channel</h4>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">[default]
exten => s,1,NoOp()
same => n,Dial(SIP/bar,,B(default^caller_handler^1))
same => n,Hangup()
exten => caller_handler,1,NoOp()
same => n,Verbose(0, In caller pre-dial handler!)
same => n,Return()</pre>
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<div class="preformatted panel" style="border-width: 1px;"><div class="preformattedHeader panelHeader" style="border-bottom-width: 1px;"><b>Example 1 CLI Output</b></div><div class="preformattedContent panelContent">
<pre><SIP/foo-123> Dial(SIP/bar,,B(default^caller_handler^1))
<SIP/foo-123> Executing default,caller_handler,1
<SIP/foo-123> In caller pre-dial handler!
<SIP/foo-123> calling SIP/bar-124
</pre>
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<h4><a name="Pre-DialHandlers-Example2Executingapredialhandleronacalleechannel"></a>Example 2 - Executing a pre-dial handler on a callee channel</h4>
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<pre class="theme: Confluence; brush: java; gutter: false">[default]
exten => s,1,NoOp()
same => n,Dial(SIP/bar,,b(default^callee_handler^1))
same => n,Hangup()
exten => callee_handler,1,NoOp()
same => n,Verbose(0, In callee pre-dial handler!)
same => n,Return()</pre>
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<div class="preformatted panel" style="border-width: 1px;"><div class="preformattedHeader panelHeader" style="border-bottom-width: 1px;"><b>Example 2 CLI Output</b></div><div class="preformattedContent panelContent">
<pre><SIP/foo-123> Dial(SIP/bar,,b(default^callee_handler^1))
<SIP/bar-124> Executing default,callee_handler,1
<SIP/bar-124> In callee pre-dial handler!
<SIP/foo-123> calling SIP/bar-124
</pre>
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<h4><a name="Pre-DialHandlers-Example3Executingapredialhandleronmultiplecalleechannels"></a>Example 3 - Executing a pre-dial handler on multiple callee channels</h4>
<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: java; gutter: false">[default]
exten => s,1,NoOp()
same => n,Dial(SIP/bar&SIP/baz,,b(default^callee_handler^1))
same => n,Hangup()
exten => callee_handler,1,NoOp()
same => n,Verbose(0, In callee pre-dial handler!)
same => n,Return()</pre>
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<div class="preformatted panel" style="border-width: 1px;"><div class="preformattedHeader panelHeader" style="border-bottom-width: 1px;"><b>Example 3 CLI Output</b></div><div class="preformattedContent panelContent">
<pre><SIP/foo-123> Dial(SIP/bar&SIP/baz,,b(default^callee_handler^1))
<SIP/bar-124> Executing default,callee_handler,1
<SIP/bar-124> In callee pre-dial handler!
<SIP/baz-125> Executing default,callee_handler,1
<SIP/baz-125> In callee pre-dial handler!
<SIP/foo-123> calling SIP/bar-124
<SIP/foo-123> calling SIP/baz-125
</pre>
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