diff --git a/libs/openzap/src/zap_isdn.c b/libs/openzap/src/zap_isdn.c
index e1f59d4565..d952aa3418 100644
--- a/libs/openzap/src/zap_isdn.c
+++ b/libs/openzap/src/zap_isdn.c
@@ -93,7 +93,7 @@ static int zap_isdn_921_21(void *pvt, L2UCHAR *msg, L2INT mlen)
 	zap_span_t *span = (zap_span_t *) pvt;
 	zap_size_t len = (zap_size_t) mlen;
 #ifdef IODEBUG
-	char bb[512] = "";
+	char bb[4096] = "";
 	print_bits(msg, (int)len, bb, sizeof(bb), 1);
 	zap_log(ZAP_LOG_DEBUG, "WRITE %d\n%s\n%s\n\n", (int)len, LINE, bb);
 
@@ -144,7 +144,7 @@ static void *zap_isdn_run(zap_thread_t *me, void *obj)
 					len = sizeof(buf);
 					if (zap_channel_read(data->dchan, buf, &len) == ZAP_SUCCESS) {
 #ifdef IODEBUG
-						char bb[512] = "";
+						char bb[4096] = "";
 						print_bits(buf, (int)len, bb, sizeof(bb), 1);
 						zap_log(ZAP_LOG_DEBUG, "READ %d\n%s\n%s\n\n", (int)len, LINE, bb);
 #endif