Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 306368)
+++ channels/chan_sip.c	(working copy)
@@ -6533,7 +6533,7 @@
 	sip_pvt_lock(i);
 	ast_channel_cc_params_init(tmp, i->cc_params);
 	tmp->caller.id.tag = ast_strdup(i->cid_tag);
-	ast_channel_unlock(tmp);
+	ast_channel_unlock(tmp); /* this lock is needed below, so deadlock avoidance is required */
 
 	tmp->tech = ( ast_test_flag(&i->flags[0], SIP_DTMF) == SIP_DTMF_INFO || ast_test_flag(&i->flags[0], SIP_DTMF) == SIP_DTMF_SHORTINFO) ?  &sip_tech_info : &sip_tech;
 
@@ -6674,6 +6674,13 @@
 		tmp->dialed.number.str = ast_strdup(i->exten);
 	}
 
+	/* pbx_builtin_setvar_helper locks the channel */
+	while (ast_channel_trylock(tmp)) {
+		sip_pvt_unlock(i);
+		sched_yield();
+		sip_pvt_lock(i);
+	}
+
 	tmp->priority = 1;
 	if (!ast_strlen_zero(i->uri))
 		pbx_builtin_setvar_helper(tmp, "SIPURI", i->uri);
@@ -6706,6 +6713,7 @@
 			"Channel: %s\r\nUniqueid: %s\r\nChanneltype: %s\r\nSIPcallid: %s\r\nSIPfullcontact: %s\r\n",
 			tmp->name, tmp->uniqueid, "SIP", i->callid, i->fullcontact);
 
+	ast_channel_unlock(tmp);
 	return tmp;
 }