Add T.38 fax support with OutboundFaxSession, InboundFaxSession, and full test coverage (rtp, sdp/types, mcp, fax)

docs/mcp.md

@@ -31,6 +31,8 @@ Set `SIP_AOR` to your SIP address-of-record.
 
 ::: voip.mcp.say
 
+::: voip.mcp.send_fax
+
 ::: voip.mcp.call
 
 [cc-mcp]: https://docs.anthropic.com/en/docs/claude-code/mcp

docs/rfc_status.md

@@ -24,6 +24,7 @@
 | [RFC 7983](https://datatracker.ietf.org/doc/html/rfc7983) | Multiplexing Scheme Updates for SRTP Extension for DTLS             | Complete | First-byte demultiplexing of STUN vs. RTP/SRTP                 |
 | [RFC 7587](https://datatracker.ietf.org/doc/html/rfc7587) | RTP Payload Format for the Opus Speech and Audio Codec              | Complete | Dynamic payload type 111                                       |
 | [RFC 3533](https://datatracker.ietf.org/doc/html/rfc3533) | The Ogg Encapsulation Format Version 0                              | Partial  | Minimal Ogg page writer for Opus audio export                  |
+| [RFC 3362](https://datatracker.ietf.org/doc/html/rfc3362) | Real-time Facsimile (T.38) — image/t38 MIME Sub-type Registration   | Complete | T.38 over SIP/UDPTL                                            |
 | [RFC 4733](https://datatracker.ietf.org/doc/html/rfc4733) | RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals | Planned  | In-band DTMF over RTP                                          |
 
 ## IVR and Application Services

docs/sessions.md

@@ -3,7 +3,8 @@
 [Session][voip.rtp.Session] and its subclasses handle the media exchange between call parties.
 They are created by the [Dialog][voip.sip.Dialog] when a call is accepted or initiated.
 
-Sessions can be audio, video, and more. However, this library currently only provides audio sessions via the [AudioCall][voip.audio.AudioCall] class. Video and other media types are fairly uncommon outside of consumer applications, and implementing them is on the roadmap but not yet a priority.
+Sessions can be audio, video, and more. This library provides audio sessions via
+[AudioCall][voip.audio.AudioCall] and T.38 FAX sessions via [FaxSession][voip.fax.FaxSession].
 
 ::: voip.rtp.Session
 
@@ -22,3 +23,15 @@ Sessions can be audio, video, and more. However, this library currently only pro
 ::: voip.ai.AgentCall
 
 ::: voip.ai.SayCall
+
+## FAX (T.38)
+
+Implements [RFC 3362] for T.38 fax over SIP/UDPTL.
+
+::: voip.fax.FaxSession
+
+::: voip.fax.OutboundFaxSession
+
+::: voip.fax.InboundFaxSession
+
+[rfc 3362]: https://datatracker.ietf.org/doc/html/rfc3362

tests/sdp/test_messages.py

@@ -759,3 +759,109 @@ def test_sample_rate__unknown_dynamic_pt__returns_none(self):
         """Dynamic PTs without an a=rtpmap have sample_rate=None (no RFC 3551 default)."""
         f = RTPPayloadFormat(payload_type=99)
         assert f.sample_rate is None
+
+
+class TestRTPPayloadFormatPostInit:
+    def test_post_init__str_payload_type__skips_static_defaults(self) -> None:
+        """String payload types (e.g. 't38') skip StaticPayloadType defaults."""
+        f = RTPPayloadFormat(payload_type="t38")
+        assert f.encoding_name is None
+        assert f.sample_rate is None
+
+
+class TestRTPPayloadFormatFrameSize:
+    def test_frame_size__static_pcmu(self) -> None:
+        """frame_size returns the RFC 3551 frame size for PCMU (PT 0)."""
+        assert RTPPayloadFormat(payload_type=0).frame_size == 160
+
+    def test_frame_size__dynamic_with_sample_rate(self) -> None:
+        """frame_size is derived from sample_rate for dynamic payload types."""
+        f = RTPPayloadFormat(
+            payload_type=111, encoding_name="opus", sample_rate=48_000, channels=2
+        )
+        assert f.frame_size == 48_000 * 20 // 1000
+
+    def test_frame_size__dynamic_without_sample_rate__uses_8000_default(self) -> None:
+        """frame_size falls back to 8000 Hz when sample_rate is None."""
+        f = RTPPayloadFormat(payload_type=99)
+        assert f.frame_size == 8_000 * 20 // 1000
+
+    def test_frame_size__str_payload_type__uses_sample_rate_fallback(self) -> None:
+        """String payload types bypass StaticPayloadType and use sample_rate fallback."""
+        f = RTPPayloadFormat(payload_type="t38")
+        assert f.frame_size == 8_000 * 20 // 1000
+
+
+class TestMediaDescriptionGetFormatNonInt:
+    def test_get_format__non_numeric_string__matches_str_payload_type(self) -> None:
+        """get_format with a non-numeric string matches str payload types."""
+        media = MediaDescription(
+            media="image",
+            port=0,
+            proto="udptl",
+            fmt=[RTPPayloadFormat(payload_type="t38")],
+        )
+        assert media.get_format("t38") is not None
+
+    def test_get_format__non_numeric_string__returns_none_when_not_found(self) -> None:
+        """get_format returns None for an unrecognised non-numeric string."""
+        media = MediaDescription(
+            media="image",
+            port=0,
+            proto="udptl",
+            fmt=[RTPPayloadFormat(payload_type="t38")],
+        )
+        assert media.get_format("vp8") is None
+
+
+class TestMediaDescriptionApplyAttributeEdgeCases:
+    def test_apply_attribute__preserves_existing_fmtp_on_rtpmap(self) -> None:
+        """Applying a=rtpmap preserves an already-set fmtp on the format."""
+        media = MediaDescription(
+            media="audio",
+            port=0,
+            proto="RTP/AVP",
+            fmt=[
+                RTPPayloadFormat(
+                    payload_type=111, fmtp="minptime=10", sample_rate=48_000
+                )
+            ],
+        )
+        media.apply_attribute(Attribute(name="rtpmap", value="111 opus/48000/2"))
+        assert media.fmt[0].fmtp == "minptime=10"
+
+    def test_apply_attribute__fmtp_non_numeric_pt__returns_false(self) -> None:
+        """apply_attribute returns False for a=fmtp with a non-numeric payload type."""
+        media = MediaDescription(
+            media="image",
+            port=0,
+            proto="udptl",
+            fmt=[RTPPayloadFormat(payload_type="t38")],
+        )
+        assert not media.apply_attribute(
+            Attribute(name="fmtp", value="t38 T38FaxVersion=0")
+        )
+
+
+class TestMediaDescriptionParseEdgeCases:
+    def test_parse__with_title(self) -> None:
+        """parse() populates title from an i= line."""
+        media = MediaDescription.parse("audio 5004 RTP/AVP 0\r\ni=Voice Channel")
+        assert media.title == "Voice Channel"
+
+    def test_parse__with_connection(self) -> None:
+        """parse() populates connection from a c= line."""
+        media = MediaDescription.parse("audio 5004 RTP/AVP 0\r\nc=IN IP4 192.0.2.1")
+        assert media.connection is not None
+        assert "192.0.2.1" in str(media.connection)
+
+    def test_parse__with_bandwidth(self) -> None:
+        """parse() appends bandwidth entries from b= lines."""
+        media = MediaDescription.parse("audio 5004 RTP/AVP 0\r\nb=AS:64")
+        assert len(media.bandwidths) == 1
+
+    def test_parse__empty_and_no_equals_lines__skipped(self) -> None:
+        """parse() skips blank lines and lines without an = sign."""
+        media = MediaDescription.parse("audio 5004 RTP/AVP 0\r\n\r\nNOEQUALS\r\n")
+        assert media.media == "audio"
+        assert media.port == 5004