Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 129493)
+++ channels/chan_sip.c	(working copy)
@@ -17449,6 +17449,8 @@
 
 	/* If we are performing an attended transfer and we have two channels involved then copy sound file information to play upon attended transfer completion */
 	if (target.chan2) {
+		ast_channel_lock(target.chan1);
+		ast_channel_lock(target.chan2);
 		const char *chan1_attended_sound = pbx_builtin_getvar_helper(target.chan1, "ATTENDED_TRANSFER_COMPLETE_SOUND"), *chan2_attended_sound = pbx_builtin_getvar_helper(target.chan2, "ATTENDED_TRANSFER_COMPLETE_SOUND");
 		if (!ast_strlen_zero(chan1_attended_sound)) {
 			pbx_builtin_setvar_helper(target.chan1, "BRIDGE_PLAY_SOUND", chan1_attended_sound);
@@ -17456,6 +17458,8 @@
 		if (!ast_strlen_zero(chan2_attended_sound)) {
 			pbx_builtin_setvar_helper(target.chan2, "BRIDGE_PLAY_SOUND", chan2_attended_sound);
 		}
+		ast_channel_unlock(target.chan1);
+		ast_channel_unlock(target.chan2);
 	}
 
 	/* Perform the transfer */