--- asterisk/channels/chan_zap.c	2005-05-16 21:15:56.000000000 +0200
+++ asterisk-patched/channels/chan_zap.c	2005-05-18 12:46:39.000000000 +0200
@@ -4954,6 +4954,13 @@
 			} else
 				break;
 		}
+		/* if no extension was received ('unspecified') on overlap call, use the 's' extension */
+		if (ast_strlen_zero(exten)) {
+			if (option_verbose > 2)
+				ast_verbose(VERBOSE_PREFIX_3 "Going to extension s|1 because of empty extension received on overlap call\n");
+			exten[0] = 's';
+			exten[1] = '\0';
+		}
 		tone_zone_play_tone(p->subs[index].zfd, -1);
 		if (ast_exists_extension(chan, chan->context, exten, 1, p->cid_num)) {
 			/* Start the real PBX */