diff --git a/api/dtmfsenderinterface.h b/api/dtmfsenderinterface.h
index b79bb31804..d8714f5d29 100644
--- a/api/dtmfsenderinterface.h
+++ b/api/dtmfsenderinterface.h
@@ -26,7 +26,14 @@ class DtmfSenderObserverInterface {
   // Triggered when DTMF |tone| is sent.
   // If |tone| is empty that means the DtmfSender has sent out all the given
   // tones.
-  virtual void OnToneChange(const std::string& tone) = 0;
+  // The callback includes the state of the tone buffer at the time when
+  // the tone finished playing.
+  virtual void OnToneChange(const std::string& tone,
+                            const std::string& tone_buffer) {}
+  // DEPRECATED: Older API without tone buffer.
+  // TODO(bugs.webrtc.org/9725): Remove old API and default implementation
+  // when old callers are gone.
+  virtual void OnToneChange(const std::string& tone) {}
 
  protected:
   virtual ~DtmfSenderObserverInterface() = default;
diff --git a/pc/dtmfsender.cc b/pc/dtmfsender.cc
index e80ad6e615..8a17b031b7 100644
--- a/pc/dtmfsender.cc
+++ b/pc/dtmfsender.cc
@@ -166,6 +166,7 @@ void DtmfSender::DoInsertDtmf() {
     tones_.clear();
     // Fire a “OnToneChange” event with an empty string and stop.
     if (observer_) {
+      observer_->OnToneChange(std::string(), tones_);
       observer_->OnToneChange(std::string());
     }
     return;
@@ -200,6 +201,8 @@ void DtmfSender::DoInsertDtmf() {
 
   // Fire a “OnToneChange” event with the tone that's just processed.
   if (observer_) {
+    observer_->OnToneChange(tones_.substr(first_tone_pos, 1),
+                            tones_.substr(first_tone_pos + 1));
     observer_->OnToneChange(tones_.substr(first_tone_pos, 1));
   }
 
diff --git a/pc/dtmfsender_unittest.cc b/pc/dtmfsender_unittest.cc
index dc8cc115b8..ce0f2d17ee 100644
--- a/pc/dtmfsender_unittest.cc
+++ b/pc/dtmfsender_unittest.cc
@@ -34,7 +34,15 @@ class FakeDtmfObserver : public DtmfSenderObserverInterface {
 
   // Implements DtmfSenderObserverInterface.
   void OnToneChange(const std::string& tone) override {
+    tones_from_single_argument_callback_.push_back(tone);
+    if (tone.empty()) {
+      completed_ = true;
+    }
+  }
+  void OnToneChange(const std::string& tone,
+                    const std::string& tone_buffer) override {
     tones_.push_back(tone);
+    tones_remaining_ = tone_buffer;
     if (tone.empty()) {
       completed_ = true;
     }
@@ -42,10 +50,16 @@ class FakeDtmfObserver : public DtmfSenderObserverInterface {
 
   // getters
   const std::vector<std::string>& tones() const { return tones_; }
+  const std::vector<std::string>& tones_from_single_argument_callback() const {
+    return tones_from_single_argument_callback_;
+  }
+  const std::string tones_remaining() { return tones_remaining_; }
   bool completed() const { return completed_; }
 
  private:
   std::vector<std::string> tones_;
+  std::vector<std::string> tones_from_single_argument_callback_;
+  std::string tones_remaining_;
   bool completed_;
 };
 
@@ -181,7 +195,10 @@ class DtmfSenderTest : public testing::Test {
     const std::vector<std::string>& tones = observer_->tones();
     // The observer will get an empty string at the end.
     EXPECT_EQ(tones_ref.size() + 1, tones.size());
+    EXPECT_EQ(observer_->tones(),
+              observer_->tones_from_single_argument_callback());
     EXPECT_TRUE(tones.back().empty());
+    EXPECT_TRUE(observer_->tones_remaining().empty());
     std::string::const_iterator it_ref = tones_ref.begin();
     std::vector<std::string>::const_iterator it = tones.begin();
     while (it_ref != tones_ref.end() && it != tones.end()) {