diff --git a/lib/amr.c b/lib/amr.c
index cf50c567b..af1c5b116 100644
--- a/lib/amr.c
+++ b/lib/amr.c
@@ -122,7 +122,11 @@ static void amr_parse_format_cb(str *key, str *token, void *data) {
 	}
 }
 static bool amr_format_parse(struct rtp_codec_format *f, const str *fmtp) {
-	codeclib_key_value_parse(fmtp, true, amr_parse_format_cb, f);
+	// Per RFC 4867 section 8.1, missing fmtp parameters use defaults (bandwidth-efficient mode, no
+	// CRC, no robust-sorting, no interleaving). Parse whatever is present. Absent fields keep their
+	// defaults.
+	if (fmtp && fmtp->len)
+		codeclib_key_value_parse(fmtp, true, amr_parse_format_cb, f);
 	return true;
 }
 static void amr_set_encdec_options(codec_options_t *opts, codec_def_t *def) {
diff --git a/lib/codeclib.c b/lib/codeclib.c
index 3c1ff2356..e65428a36 100644
--- a/lib/codeclib.c
+++ b/lib/codeclib.c
@@ -149,8 +149,6 @@ bool codec_parse_fmtp(codec_def_t *def, struct rtp_codec_format *fmtp, const str
 		return false;
 	if (!def->format_parse)
 		return true;
-	if (!fmtp_string)
-		return true;
 	if (!fmtp) {
 		ZERO(fmtp_store);
 		fmtp = &fmtp_store;
@@ -160,6 +158,8 @@ bool codec_parse_fmtp(codec_def_t *def, struct rtp_codec_format *fmtp, const str
 			*copy = fmtp->parsed;
 		return true;
 	}
+	// Call format_parse even without fmtp_string so that codecs can populate defaults and set
+	// fmtp_parsed = true (e.g. AMR with RFC 4867 defaults).
 	bool ret = def->format_parse(fmtp, fmtp_string);
 	if (ret) {
 		fmtp->fmtp_parsed = true;
diff --git a/lib/ilbc.c b/lib/ilbc.c
index 7fd13efb6..c1c96af09 100644
--- a/lib/ilbc.c
+++ b/lib/ilbc.c
@@ -3,6 +3,8 @@
 
 
 static bool ilbc_format_parse(struct rtp_codec_format *f, const str *fmtp) {
+	if (!fmtp || !fmtp->len)
+		return false;
 	switch (__csh_lookup(fmtp)) {
 		case CSH_LOOKUP("mode=20"):
 			f->parsed.ilbc.mode = 20;
diff --git a/t/auto-daemon-tests-unknown-fmtp.pl b/t/auto-daemon-tests-unknown-fmtp.pl
index 272dd99df..c6fb10a54 100644
--- a/t/auto-daemon-tests-unknown-fmtp.pl
+++ b/t/auto-daemon-tests-unknown-fmtp.pl
@@ -207,4 +207,196 @@
 SDP
 
 #done_testing;NGCP::Rtpengine::AutoTest::terminate('f00');exit;
+
+# AMR-WB with octet-align=0 in offer, answer without fmtp (defaults match)
+new_call;
+
+offer('AMR-WB octet-align=0 offer, answer missing fmtp - should match', {
+    'rtpp-flags' => 'replace-origin address-family=IP4 transport-protocol=RTP/AVP',
+    'from-tag' => ft(),
+    }, <<SDP);
+v=0
+o=- 1545997027 1 IN IP4 198.51.100.1
+s=tester
+t=0 0
+m=audio 2000 RTP/AVP 96 101
+a=rtpmap:96 AMR-WB/16000
+a=fmtp:96 octet-align=0
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+c=IN IP4 198.51.100.1
+a=sendrecv
+----------------------------------
+v=0
+o=- 1545997027 1 IN IP4 203.0.113.1
+s=tester
+t=0 0
+m=audio PORT RTP/AVP 96 101
+c=IN IP4 203.0.113.1
+a=rtpmap:96 AMR-WB/16000
+a=fmtp:96 octet-align=0
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+a=sendrecv
+a=rtcp:PORT
+SDP
+
+# Answer without fmtp: per RFC 4867 s8.1, missing fmtp = defaults = octet-align=0
+# This should match the offer's octet-align=0
+answer('AMR-WB octet-align=0 offer, answer missing fmtp - should match', {
+    'rtpp-flags' => 'replace-origin address-family=IP4 transport-protocol=RTP/AVP',
+    'from-tag' => ft(),
+    'to-tag' => tt(),
+    }, <<SDP);
+v=0
+o=- 1545997027 1 IN IP4 198.51.100.3
+s=tester
+t=0 0
+m=audio 2002 RTP/AVP 96 101
+a=rtpmap:96 AMR-WB/16000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+c=IN IP4 198.51.100.3
+a=sendrecv
+--------------------------------------
+v=0
+o=- 1545997027 1 IN IP4 203.0.113.1
+s=tester
+t=0 0
+m=audio PORT RTP/AVP 96 101
+c=IN IP4 203.0.113.1
+a=rtpmap:96 AMR-WB/16000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+a=sendrecv
+a=rtcp:PORT
+SDP
+
+# AMR-WB with octet-align=1 in offer, answer without fmtp (incompatible)
+new_call;
+
+offer('AMR-WB octet-align=1 offer, answer missing fmtp - incompatible', {
+    'rtpp-flags' => 'replace-origin address-family=IP4 transport-protocol=RTP/AVP',
+    'from-tag' => ft(),
+    }, <<SDP);
+v=0
+o=- 1545997027 1 IN IP4 198.51.100.1
+s=tester
+t=0 0
+m=audio 2000 RTP/AVP 96 0 101
+a=rtpmap:96 AMR-WB/16000
+a=fmtp:96 octet-align=1
+a=rtpmap:0 PCMU/8000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+c=IN IP4 198.51.100.1
+a=sendrecv
+----------------------------------
+v=0
+o=- 1545997027 1 IN IP4 203.0.113.1
+s=tester
+t=0 0
+m=audio PORT RTP/AVP 96 0 101
+c=IN IP4 203.0.113.1
+a=rtpmap:96 AMR-WB/16000
+a=fmtp:96 octet-align=1
+a=rtpmap:0 PCMU/8000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+a=sendrecv
+a=rtcp:PORT
+SDP
+
+# Answer without fmtp: missing = defaults = octet-align=0, which != octet-align=1
+# AMR-WB should be rejected, PCMU should remain
+answer('AMR-WB octet-align=1 offer, answer missing fmtp - incompatible', {
+    'rtpp-flags' => 'replace-origin address-family=IP4 transport-protocol=RTP/AVP',
+    'from-tag' => ft(),
+    'to-tag' => tt(),
+    }, <<SDP);
+v=0
+o=- 1545997027 1 IN IP4 198.51.100.3
+s=tester
+t=0 0
+m=audio 2002 RTP/AVP 96 0 101
+a=rtpmap:96 AMR-WB/16000
+a=rtpmap:0 PCMU/8000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+c=IN IP4 198.51.100.3
+a=sendrecv
+--------------------------------------
+v=0
+o=- 1545997027 1 IN IP4 203.0.113.1
+s=tester
+t=0 0
+m=audio PORT RTP/AVP 0 101
+c=IN IP4 203.0.113.1
+a=rtpmap:0 PCMU/8000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+a=sendrecv
+a=rtcp:PORT
+SDP
+
+# AMR-WB with no fmtp in both offer and answer (both use defaults, should match)
+new_call;
+
+offer('AMR-WB no fmtp in offer or answer - defaults match', {
+    'rtpp-flags' => 'replace-origin address-family=IP4 transport-protocol=RTP/AVP',
+    'from-tag' => ft(),
+    }, <<SDP);
+v=0
+o=- 1545997027 1 IN IP4 198.51.100.1
+s=tester
+t=0 0
+m=audio 2000 RTP/AVP 96 101
+a=rtpmap:96 AMR-WB/16000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+c=IN IP4 198.51.100.1
+a=sendrecv
+----------------------------------
+v=0
+o=- 1545997027 1 IN IP4 203.0.113.1
+s=tester
+t=0 0
+m=audio PORT RTP/AVP 96 101
+c=IN IP4 203.0.113.1
+a=rtpmap:96 AMR-WB/16000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+a=sendrecv
+a=rtcp:PORT
+SDP
+
+answer('AMR-WB no fmtp in offer or answer - defaults match', {
+    'rtpp-flags' => 'replace-origin address-family=IP4 transport-protocol=RTP/AVP',
+    'from-tag' => ft(),
+    'to-tag' => tt(),
+    }, <<SDP);
+v=0
+o=- 1545997027 1 IN IP4 198.51.100.3
+s=tester
+t=0 0
+m=audio 2002 RTP/AVP 96 101
+a=rtpmap:96 AMR-WB/16000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+c=IN IP4 198.51.100.3
+a=sendrecv
+--------------------------------------
+v=0
+o=- 1545997027 1 IN IP4 203.0.113.1
+s=tester
+t=0 0
+m=audio PORT RTP/AVP 96 101
+c=IN IP4 203.0.113.1
+a=rtpmap:96 AMR-WB/16000
+a=rtpmap:101 telephone-event/8000
+a=fmtp:101 0-15
+a=sendrecv
+a=rtcp:PORT
+SDP
+
 done_testing();
diff --git a/t/test-transcode.c b/t/test-transcode.c
index ae3d39b24..d0611d9be 100644
--- a/t/test-transcode.c
+++ b/t/test-transcode.c
@@ -119,6 +119,7 @@ static void __start(const char *file, int line) {
 }
 
 #define transcode(codec) t_queue_push_tail(&flags.codec_transcode, sdup(#codec))
+#define transcode_s(s) t_queue_push_tail(&flags.codec_transcode, sdup(s))
 #define c_accept(codec) t_queue_push_tail(&flags.codec_accept, sdup(#codec))
 #define c_consume(codec) t_queue_push_tail(&flags.codec_consume, sdup(#codec))
 #define c_mask(codec) t_queue_push_tail(&flags.codec_mask, sdup(#codec))
@@ -621,6 +622,49 @@ int main(void) {
 			packet_seq(A, 96, AMR_WB_payload_noe, 0, 0, -1, ""); // nothing due to resampling/decoding buffer
 			packet_seq_nf(A, 96, AMR_WB_payload_noe, 320, 1, 0, PCMU_payload);
 			end();
+
+			// forward AMR-WB, answer without fmtp (missing = defaults = octet-align=0)
+			// offer has octet-align=0, answer omits fmtp -> should match
+			start();
+			sdp_pt(0, PCMU, 8000);
+			transcode_s("AMR-WB/16000/1///octet-align=0");
+			offer();
+			expect(A, "0/PCMU/8000");
+			expect(B, "0/PCMU/8000 96/AMR-WB/16000/octet-align=0");
+			sdp_pt(96, AMR-WB, 16000); // answer without fmtp
+			answer();
+			expect(A, "0/PCMU/8000");
+			expect(B, "96/AMR-WB/16000");
+			end();
+
+			// forward AMR-WB, answer without fmtp but offer has octet-align=1
+			// these are incompatible
+			start();
+			sdp_pt(0, PCMU, 8000);
+			transcode(AMR-WB);
+			offer();
+			expect(A, "0/PCMU/8000");
+			expect(B, "0/PCMU/8000 96/AMR-WB/16000/octet-align=1;mode-change-capability=2");
+			sdp_pt(0, PCMU, 8000);
+			sdp_pt(96, AMR-WB, 16000); // answer with PCMU + AMR-WB (no fmtp implies octet-align=0)
+			answer();
+			// AMR-WB should be rejected as incompatible (octet-align=0 != octet-align=1), only PCMU remains
+			expect(A, "0/PCMU/8000");
+			expect(B, "0/PCMU/8000");
+			end();
+
+			// forward AMR-WB, answer with matching octet-align=1
+			start();
+			sdp_pt(0, PCMU, 8000);
+			transcode(AMR-WB);
+			offer();
+			expect(A, "0/PCMU/8000");
+			expect(B, "0/PCMU/8000 96/AMR-WB/16000/octet-align=1;mode-change-capability=2");
+			sdp_pt_fmt(96, AMR-WB, 16000, "octet-align=1");
+			answer();
+			expect(A, "0/PCMU/8000");
+			expect(B, "96/AMR-WB/16000/octet-align=1");
+			end();
 		}
 	}