Index: main/pbx.c
===================================================================
--- main/pbx.c	(revision 349445)
+++ main/pbx.c	(working copy)
@@ -719,6 +719,35 @@
 			<ref type="application">RaiseException</ref>
 		</see-also>
 	</function>
+	<function name="REGISTRANT" language="en_US">
+		<synopsis>
+			Retrieve the SIP peer that auto-registered an extension.
+		</synopsis>
+		<syntax argsep="@">
+			<parameter name="extension" required="false">
+				<para>Default is the current extension.</para>
+				<para>Priority 1 of <replaceable>extension</replaceable> should have been
+				auto-registered by SIP, possibly via <literal>regexten=</literal>.</para>
+			</parameter>
+			<parameter name="context" required="false">
+				<para>The context to use when searching for <replaceable>extension</replaceable>.
+				Default is the current context.</para>
+				<para><replaceable>context</replaceable> should be one specified by <literal>regcontext=</literal> in
+				sip.conf.</para>
+			</parameter>
+		</syntax>
+		<description>
+			<para>If <replaceable>extension</replaceable> was auto-registered in <replaceable>context</replaceable> as the
+			result of <literal>regcontext=</literal> in sip.conf, REGISTRANT returns the name of its associated peer, even if
+			<replaceable>extension</replaceable> was created due to <literal>regexten=</literal>.</para>
+			<para>If priority 1 of <replaceable>extension</replaceable> can't be found, or wasn't created by SIP, the
+			null string is returned.</para>
+			<para> Example:  Here, <literal>regexten=</literal> has been used to assign extensions in the range 250-299 to
+			peers with names like 002699ABD834.</para>
+			<para>   include => sip-autoreg    ; regcontext = sip-autoreg</para>
+			<para>   exten => _2[5-9]X, 2, Dial(SIP/${REGISTRANT()})</para>
+		</description>
+	</function>
 	<function name="TESTTIME" language="en_US">
 		<synopsis>
 			Sets a time to be used with the channel to test logical conditions.
@@ -3474,6 +3503,38 @@
 	.read = acf_exception_read,
 };
 
+static int acf_registrant(struct ast_channel *chan, const char *name, char *data, char *buf, size_t len)
+{
+	char *context, *exten;
+	struct ast_exten *e;
+	struct pbx_find_info q = { .stacklen = 0 };
+	char *parse;
+	AST_DECLARE_APP_ARGS(args,
+		AST_APP_ARG(exten);
+		AST_APP_ARG(context);
+	);
+
+	parse = ast_strdupa(data);
+	AST_NONSTANDARD_APP_ARGS(args, parse, '@');
+	exten = ast_strlen_zero(args.exten) ? chan->exten : args.exten;
+	context = ast_strlen_zero(args.context) ? chan->context : args.context;
+	buf[0] = '\0';
+	e = pbx_find_extension(chan, NULL, &q, context, exten, 1, NULL, NULL, E_MATCH);
+	if (e							/* Did we find exten, 1 ?	*/
+	    && e->registrar && !strcmp(e->registrar, "SIP")	/* Registered by SIP ?		*/
+	    && e->app && !strcmp(e->app, "Noop")		/* Does it execute Noop() ?	*/
+	    && !ast_strlen_zero(e->data)) {			/* And Noop() has an argument ?	*/
+		ast_copy_string(buf, e->data, len);		/* That's our peer		*/
+		return 0;
+	}
+	return -1;
+}
+
+static struct ast_custom_function registrant = {
+	.name = "REGISTRANT",
+	.read = acf_registrant,
+};
+
 static char *handle_show_functions(struct ast_cli_entry *e, int cmd, struct ast_cli_args *a)
 {
 	struct ast_custom_function *acf;
@@ -10449,6 +10510,7 @@
 	ast_data_register_multiple_core(pbx_data_providers, ARRAY_LEN(pbx_data_providers));
 	__ast_custom_function_register(&exception_function, NULL);
 	__ast_custom_function_register(&testtime_function, NULL);
+	__ast_custom_function_register(&registrant, NULL);
 
 	/* Register builtin applications */
 	for (x = 0; x < ARRAY_LEN(builtins); x++) {