diff --git a/libs/openzap/src/isdn/Q921.c b/libs/openzap/src/isdn/Q921.c
index 944a8a5a31..7b084fe230 100644
--- a/libs/openzap/src/isdn/Q921.c
+++ b/libs/openzap/src/isdn/Q921.c
@@ -504,7 +504,7 @@ int Q921Rx12(L2TRUNK trunk)
 
 			/* we increment before we "really" know its good so that if we send in the callback, we use the right nr */
 			trunk->vr++;
-            if(Q921Tx23Proc(trunk, smes, size-2) >= 0) /* -2 to clip away CRC */
+            if(Q921Tx23Proc(trunk, smes, size-2)) /* -2 to clip away CRC */
             {
 				Q921SendRR(trunk, (mes[0]&0xfc)>>2, (mes[0]>>1)&0x01, mes[1]>>1, mes[3]&0x01);
 			}
diff --git a/libs/openzap/src/zap_isdn.c b/libs/openzap/src/zap_isdn.c
index 2ac84395e5..339dd02a73 100644
--- a/libs/openzap/src/zap_isdn.c
+++ b/libs/openzap/src/zap_isdn.c
@@ -88,6 +88,11 @@ static L3INT zap_isdn_931_34(void *pvt, L2UCHAR *msg, L2INT mlen)
 	return 0;
 }
 
+static int zap_isdn_921_23(void *pvt, L2UCHAR *msg, L2INT mlen)
+{
+	return ((Q931Rx23(pvt, msg, mlen) >= 0) ? 1 : 0);
+}
+
 static int zap_isdn_921_21(void *pvt, L2UCHAR *msg, L2INT mlen)
 {
 	zap_span_t *span = (zap_span_t *) pvt;
@@ -235,7 +240,7 @@ zap_status_t zap_isdn_configure_span(zap_span_t *span, Q921NetUser_t mode, Q931D
 				   mode,
 				   0,
 				   zap_isdn_921_21,
-				   (Q921TxCB_t)Q931Rx23,
+				   (Q921TxCB_t)zap_isdn_921_23,
 				   span,
 				   &span->isdn_data->q931);