Index: channels/sig_pri.c
===================================================================
--- channels/sig_pri.c	(revision 257187)
+++ channels/sig_pri.c	(working copy)
@@ -4264,7 +4264,15 @@
 	l = NULL;
 	n = NULL;
 	if (!p->hidecallerid) {
-		l = ast->connected.id.number;
+		/* If we get to the end of this loop without breaking, there's no calleridnum.
+		 * This is done instead of testing for "unknown" or the thousands of other ways
+		 * that the calleridnum could be invalid. */
+		for (l = ast->connected.id.number; l && *l; l++) {
+			if (strchr("0123456789", *l)) {
+				l = ast->connected.id.number;
+				break;
+			}
+		}
 		if (!p->hidecalleridname) {
 			n = ast->connected.id.name;
 		}