diff --git a/openbsc/include/openbsc/mgcp_internal.h b/openbsc/include/openbsc/mgcp_internal.h
index b16bd49..255161f 100644
--- a/openbsc/include/openbsc/mgcp_internal.h
+++ b/openbsc/include/openbsc/mgcp_internal.h
@@ -85,7 +85,7 @@
 	char *fmtp_extra;
 
 	/* RTP patching */
-	int force_constant_ssrc;
+	int force_constant_ssrc; /* -1: always, 0: don't, 1: once */
 	int force_constant_timing;
 
 	/*
diff --git a/openbsc/src/libmgcp/mgcp_network.c b/openbsc/src/libmgcp/mgcp_network.c
index abb48f5..79c9e1a 100644
--- a/openbsc/src/libmgcp/mgcp_network.c
+++ b/openbsc/src/libmgcp/mgcp_network.c
@@ -280,6 +280,8 @@
 				timestamp;
 			state->patch = 1;
 			ssrc = state->orig_ssrc;
+			if (rtp_end->force_constant_ssrc != -1)
+				rtp_end->force_constant_ssrc -= 1;
 
 			LOGP(DMGCP, LOGL_NOTICE,
 			     "SSRC patching enabled on 0x%x SSRC: %u "
diff --git a/openbsc/src/libmgcp/mgcp_protocol.c b/openbsc/src/libmgcp/mgcp_protocol.c
index c703bec..b8e1ecd 100644
--- a/openbsc/src/libmgcp/mgcp_protocol.c
+++ b/openbsc/src/libmgcp/mgcp_protocol.c
@@ -619,8 +619,10 @@
 {
 	struct mgcp_trunk_config *tcfg = endp->tcfg;
 
+	int patch_ssrc = expect_ssrc_change && tcfg->force_constant_ssrc;
+
 	rtp->force_constant_timing = tcfg->force_constant_timing;
-	rtp->force_constant_ssrc = tcfg->force_constant_ssrc;
+	rtp->force_constant_ssrc = patch_ssrc ? 1 : 0;
 
 	LOGP(DMGCP, LOGL_DEBUG,
 	     "Configuring RTP endpoint: local port %d%s%s\n",