diff --git a/lib/codeclib.c b/lib/codeclib.c
index e02f1e32c..2fef4f855 100644
--- a/lib/codeclib.c
+++ b/lib/codeclib.c
@@ -516,7 +516,7 @@ static struct codec_def_s __codec_defs[] = {
 		.default_channels = 1,
 		.default_ptime = 20,
 		.minimum_ptime = 20,
-		.default_fmtp = "annexb=no",
+		.default_fmtp = "annexb=yes",
 		.format_cmp = format_cmp_g729,
 		.packetizer = packetizer_g729,
 		.bits_per_sample = 1, // 10 ms frame has 80 samples and encodes as (max) 10 bytes = 80 bits
@@ -535,6 +535,7 @@ static struct codec_def_s __codec_defs[] = {
 		.default_channels = 1,
 		.default_ptime = 20,
 		.minimum_ptime = 20,
+		.default_fmtp = "annexb=no",
 		.format_cmp = format_cmp_g729,
 		.packetizer = packetizer_g729,
 		.bits_per_sample = 1, // 10 ms frame has 80 samples and encodes as (max) 10 bytes = 80 bits
@@ -3623,19 +3624,20 @@ static int packetizer_g729(AVPacket *pkt, GString *buf, str *input_output, encod
 	return buf->len >= 2 ? 1 : 0;
 }
 
+static bool g729_is_annex_b(const struct rtp_payload_type *p) {
+	// defaults based on codec name (G729 vs G729a)
+	bool annex_b = p->encoding.len && !((p->encoding.s[p->encoding.len - 1] & 0xdf) == 'A');
+	// override per fmtp
+	if (str_str(&p->format_parameters, "annexb=no") != -1)
+		annex_b = false;
+	else if (str_str(&p->format_parameters, "annexb=yes") != -1)
+		annex_b = true;
+	return annex_b;
+}
 static int format_cmp_g729(const struct rtp_payload_type *a, const struct rtp_payload_type *b) {
-	// shortcut the most common case:
-	if (!str_cmp_str(&a->format_parameters, &b->format_parameters))
-		return 0;
-	// incompatible is if one side uses annex B but the other one doesn't
-	if (str_str(&a->format_parameters, "annexb=yes") != -1
-			&& str_str(&b->format_parameters, "annexb=yes") == -1)
-		return -1;
-	if (str_str(&a->format_parameters, "annexb=yes") == -1
-			&& str_str(&b->format_parameters, "annexb=yes") != -1)
-		return -1;
-	// everything else is compatible
-	return 0;
+	bool a_b = g729_is_annex_b(a);
+	bool b_b = g729_is_annex_b(b);
+	return a_b == b_b ? 0 : -1;
 }
 #endif
 
diff --git a/t/auto-daemon-tests.pl b/t/auto-daemon-tests.pl
index 0b66b4dce..639c997f1 100755
--- a/t/auto-daemon-tests.pl
+++ b/t/auto-daemon-tests.pl
@@ -538,7 +538,7 @@ m=audio 49696 RTP/AVP 8 0 18 100
 a=rtpmap:8 PCMA/8000
 a=rtpmap:0 PCMU/8000
 a=rtpmap:18 G729/8000
-a=fmtp:18 annexb=no
+a=fmtp:18 annexb=yes
 a=rtpmap:100 telephone-event/8000
 a=fmtp:100 0-11
 a=ptime:20
@@ -555,7 +555,7 @@ a=maxptime:30
 a=rtpmap:8 PCMA/8000
 a=rtpmap:0 PCMU/8000
 a=rtpmap:18 G729/8000
-a=fmtp:18 annexb=no
+a=fmtp:18 annexb=yes
 a=rtpmap:100 telephone-event/8000
 a=fmtp:100 0-11
 a=sendrecv
@@ -601,7 +601,7 @@ m=audio 49696 RTP/AVP 8 0 18 100
 a=rtpmap:8 PCMA/8000
 a=rtpmap:0 PCMU/8000
 a=rtpmap:18 G729/8000
-a=fmtp:18 annexb=yes
+a=fmtp:18 annexb=no
 a=rtpmap:100 telephone-event/8000
 a=fmtp:100 0-11
 a=ptime:20
@@ -674,11 +674,10 @@ o=- 1737116508926565 1737116508926565 IN IP4 5.6.7.7
 s=SIP call
 c=IN IP4 203.0.113.1
 t=0 0
-m=audio PORT RTP/AVP 8 0 18 100
+m=audio PORT RTP/AVP 8 0 100
 a=maxptime:30
 a=rtpmap:8 PCMA/8000
 a=rtpmap:0 PCMU/8000
-a=rtpmap:18 G729a/8000
 a=rtpmap:100 telephone-event/8000
 a=fmtp:100 0-11
 a=sendrecv
@@ -736,12 +735,10 @@ o=- 1737116508926565 1737116508926565 IN IP4 5.6.7.7
 s=SIP call
 c=IN IP4 203.0.113.1
 t=0 0
-m=audio PORT RTP/AVP 8 0 18 100
+m=audio PORT RTP/AVP 8 0 100
 a=maxptime:30
 a=rtpmap:8 PCMA/8000
 a=rtpmap:0 PCMU/8000
-a=rtpmap:18 G729a/8000
-a=fmtp:18 annexb=no
 a=rtpmap:100 telephone-event/8000
 a=fmtp:100 0-11
 a=sendrecv
@@ -787,7 +784,7 @@ m=audio 49696 RTP/AVP 8 0 18 100
 a=rtpmap:8 PCMA/8000
 a=rtpmap:0 PCMU/8000
 a=rtpmap:18 G729a/8000
-a=fmtp:18 annexb=yes
+a=fmtp:18 annexb=no
 a=rtpmap:100 telephone-event/8000
 a=fmtp:100 0-11
 a=ptime:20