when far end answers with only pcma, passthrough instead of transcoding to pcmu (#208)

.gitignore

@@ -36,3 +36,4 @@ node_modules
 .DS_Store
 
 examples/*
+CLAUDE.md

lib/call-session.js

@@ -9,7 +9,8 @@ const {
   makeFullMediaReleaseKey,
   makePartnerFullMediaReleaseKey,
   isValidDomainOrIP,
-  removeVideoSdp
+  removeVideoSdp,
+  determineAnswerCodec
 } = require('./utils');
 const { MediaPath } = require('./constants.json');
 const {forwardInDialogRequests} = require('drachtio-fn-b2b-sugar');
@@ -99,8 +100,9 @@ const updateRtpEngineFlags = (sdp, opts) => {
   try {
     const parsed = sdpTransform.parse(sdp);
     const codec = parsed.media[0].rtp[0].codec;
+    // Only add telephone-event support, don't restrict to specific G.711 codec yet
+    // This allows far end to choose between PCMU/PCMA freely
     if (['PCMU', 'PCMA'].includes(codec)) {
-      opts.flags.push(`codec-accept-${codec}`);
       opts.flags.push('codec-accept-telephone-event');
     }
   } catch {}
@@ -554,12 +556,16 @@ class CallSession extends Emitter {
             localSdpB: response.sdp,
             localSdpA: async(sdp, res) => {
               this.rtpEngineOpts.uac.tag = res.getParsedHeader('To').params.tag;
+
+              // Determine which codec to use based on far end negotiation
+              const {codec} = determineAnswerCodec(sdp, this.req.body, this.logger);
+
               const opts = {
                 ...this.rtpEngineOpts.common,
                 ...this.rtpEngineOpts.uas.mediaOpts,
                 'from-tag': this.rtpEngineOpts.uas.tag,
                 'to-tag': this.rtpEngineOpts.uac.tag,
-                flags: ['single codec', 'inject DTMF'],
+                flags: ['single codec', 'inject DTMF', `codec-accept-${codec}`, 'codec-accept-telephone-event'],
                 sdp
               };
               const response = await this.answer(opts);

lib/utils.js

@@ -349,6 +349,55 @@ const removeVideoSdp = (sdp) => {
   parsedSdp.media = parsedSdp.media.filter((media) => media.type !== 'video');
   return sdpTransform.write(parsedSdp);
 };
+
+const determineAnswerCodec = (farEndSdp, featureServerSdp, logger) => {
+  try {
+    // Parse both SDPs
+    const farEndParsed = sdpTransform.parse(farEndSdp);
+    const fsParsed = sdpTransform.parse(featureServerSdp);
+
+    // Get negotiated codec from far end (first codec in answer)
+    const negotiatedCodec = farEndParsed.media[0].rtp[0].codec;
+
+    // Get all codecs offered by feature server
+    const fsCodecs = fsParsed.media[0].rtp.map((r) => r.codec);
+
+    logger.debug({negotiatedCodec, fsCodecs}, 'determineAnswerCodec: analyzing codec negotiation');
+
+    // If far end negotiated G.711 (PCMU/PCMA) AND it was in the FS offer, pass it through
+    if (['PCMU', 'PCMA'].includes(negotiatedCodec) && fsCodecs.includes(negotiatedCodec)) {
+      logger.info({negotiatedCodec}, 'G.711 codec passthrough - no transcoding needed');
+      return {
+        codec: negotiatedCodec,
+        needsTranscoding: false
+      };
+    }
+
+    // Otherwise, we need to transcode to first G.711 codec in FS offer
+    const firstG711 = fsCodecs.find((c) => ['PCMU', 'PCMA'].includes(c));
+    if (firstG711) {
+      logger.info({negotiatedCodec, transcodeTarget: firstG711}, 'Transcoding required to G.711');
+      return {
+        codec: firstG711,
+        needsTranscoding: true
+      };
+    }
+
+    // Fallback: use PCMU
+    logger.info({negotiatedCodec}, 'No G.711 in FS offer, defaulting to PCMU');
+    return {
+      codec: 'PCMU',
+      needsTranscoding: true
+    };
+  } catch (err) {
+    logger.error({err}, 'Error determining answer codec, defaulting to PCMU');
+    return {
+      codec: 'PCMU',
+      needsTranscoding: true
+    };
+  }
+};
+
 module.exports = {
   makeRtpEngineOpts,
   selectHostPort,
@@ -364,5 +413,6 @@ module.exports = {
   makeFullMediaReleaseKey,
   makePartnerFullMediaReleaseKey,
   isValidDomainOrIP,
-  removeVideoSdp
+  removeVideoSdp,
+  determineAnswerCodec
 };