Top Libraries for Creating a C# Softphone With Call Recording

Developing a custom C# softphone with call recording involves integrating a Session Initiation Protocol (SIP) stack to handle signaling and utilizing an audio framework to capture and mix the voice streams. To avoid rewriting lower-level network and media processing code from scratch, developers primarily use specialized toolkits like the Ozeki VoIP SIP SDK or open-source wrappers around libraries like PJSIP. Core Architectural Components

Building a functional softphone requires managing two distinct network layers alongside your application interface:

SIP Signaling: Handles user registration with a VoIP PBX, call initiation (INVITE), hanging up (BYE), and state changes.

Media Real-Time Transport Protocol (RTP): Handles encoding, decoding, and streaming of actual voice data over the network using audio codecs.

Audio Processing (The Recorder): Intercepts the microphone input stream and the incoming remote speaker stream, channels them through a mixer, and writes them to a file. Step-by-Step Implementation Guide 1. Set Up Your Project environment

Create a new C# Windows Forms or WPF Application in Microsoft Visual Studio. You must import the libraries that will manage the SIP stack. If you are using a commercial toolkit like Ozeki, right-click References, click Add Reference, and browse to the SDK’s DLL file (e.g., VoIPSDK.dll). 2. Initialize the Softphone and Register SIP Account

Instantiate your phone object and supply the credentials needed to connect to your VoIP PBX or service provider.

using Ozeki.VoIP; using Ozeki.Media; public class SoftphoneManager { private ISoftPhone _softPhone; private IPhoneLine _phoneLine; private IPhoneCall _currentCall; public void Initialize() { // Initialize softphone bound to a local IP and local port range _softPhone = SoftPhoneFactory.CreateSoftPhone(SoftPhoneFactory.GetLocalIP(), 5700, 5750); // Listen for incoming calls _softPhone.IncomingCall += SoftPhone_IncomingCall; // Set up SIP credentials SIPAccount account = new SIPAccount(true, “user123”, “displayName”, “authName”, “password”, “pbx_ip_address”, 5060); _phoneLine = _softPhone.CreatePhoneLine(account); // Register to the PBX _softPhone.RegisterPhoneLine(_phoneLine); } } Use code with caution. 3. Manage Call States and Audio Routing

You must hook into the call status events to route audio when a call transitions into an active talking state.

private void SoftPhone_IncomingCall(object sender, VoIPEventArgs e) { _currentCall = e.Item; _currentCall.CallStateChanged += Call_CallStateChanged; } private void Call_CallStateChanged(object sender, VoIPEventArgs e) { if (e.Item == CallState.InCall) { // Call is active; connect the audio devices and start recording StartAudioAndRecording(); } else if (e.Item == CallState.Completed) { // Call ended; stop recording components StopRecording(); } } Use code with caution. 4. Implement Call Recording and Audio Mixing

To capture both sides of the conversation cleanly without distortion, you cannot simply record from the microphone. You must route the microphone stream (outgoing audio) and the media receiver stream (incoming audio) into an Audio Mixer, then link that mixer directly to an audio recorder.

private AudioMixerMediaHandler _audioMixer; private WaveStreamRecorder _waveRecorder; private Microphone _microphone; private Speaker _speaker; private void StartAudioAndRecording() { // Initialize default audio hardware _microphone = Microphone.GetDefaultDevice(); _speaker = Speaker.GetDefaultDevice(); _audioMixer = new AudioMixerMediaHandler(); waveRecorder = new WaveStreamRecorder(@“C:\Calls\RecordedCall” + DateTime.Now.ToString(“yyyyMMdd_HHmmss”) + “.wav”); // Connect hardware devices to the call session _microphone.Connect(_currentCall.AudioSender); _currentCall.AudioReceiver.Connect(_speaker); // Route both paths into the mixer for simultaneous call recording _microphone.Connect(_audioMixer); _currentCall.AudioReceiver.Connect(_audioMixer); // Connect the mixed audio to the wave recorder and start writing to disk _audioMixer.Connect(_waveRecorder); _microphone.Start(); _speaker.Start(); _waveRecorder.Start(); // Recording begins here } private void StopRecording() { _waveRecorder.Stop(); _microphone.Stop(); _speaker.Stop(); // Clean up and disconnect handlers _microphone.Disconnect(_currentCall.AudioSender); _currentCall.AudioReceiver.Disconnect(_speaker); _microphone.Disconnect(_audioMixer); _currentCall.AudioReceiver.Disconnect(_audioMixer); _audioMixer.Disconnect(_waveRecorder); _waveRecorder.Dispose(); _audioMixer.Dispose(); } Use code with caution. Crucial Technical Design Considerations How to record voice calls (C#, SIP) – DZone