Building a C# SIP Softphone: A Step-by-Step Tutorial

How to Create a SIP Softphone in C#: Complete Code ExampleCreating a SIP (Session Initiation Protocol) softphone using C# is an exciting project that combines telecommunications with software development. This article will guide you through the process of developing a SIP softphone, complete with a code example, which will allow you to make and receive VoIP calls.

Understanding SIP and Softphones

SIP is a signaling protocol used for initiating, maintaining, and terminating real-time sessions in VoIP (Voice over IP). A softphone is software that enables voice communication over the internet through a computer or mobile device, acting as a virtual phone.

Prerequisites

Before you start coding, make sure you have the following tools and libraries:

1. Visual Studio: An IDE for developing C# applications.
2. PJSIP: A free and open-source multimedia communication library that implements standard-based protocols, including SIP.
3. .NET Framework: Make sure you are familiar with C# and the .NET ecosystem.

Setting Up Your Environment

1. Install Visual Studio: Download and install Visual Studio if it’s not already installed on your system.
2. Download PJSIP: You can get the PJSIP library from its official website. Follow their documentation for installation instructions.

Project Structure

Create a new C# Console Application in Visual Studio. Your project will have the following structure:

SIPSoftphone/ ├── Program.cs ├── SIPClient.cs └── resources/     └── config.xml // (if needed for configuration settings) 

Implementing the SIP Client

Below is a simplified implementation of a SIP softphone using C#. We will go through the main components step by step.

SIPClient.cs

Create a new class called SIPClient.cs and implement the following methods:

using System; using System.Runtime.InteropServices; using PJSUA2; // Ensure you have the PJSUA2 installed and referenced. public class SIPClient {     private Endpoint ep;     private Account account;     public SIPClient()     {         // Create the PJSIP endpoint         ep = new Endpoint();         ep.libCreate();                  // Initialize the PJSIP library         var epConfig = new EpConfig();         ep.libInit(epConfig);                  // Create the transport         var transportConfig = new TransportConfig();         transportConfig.port = 5060; // Default SIP port         ep.transportCreate(TransportType.UDP, transportConfig);         // Start the library         ep.libStart();         // Initialize the account         account = new Account();     }     public void Register(string id, string password, string domain)     {         var accConfig = new AccountConfig();         accConfig.idUri = id;         accConfig.regConfig.registrarUri = domain;         accConfigcred.userName = id;         accConfig.cred.password = password;         account.Create(accConfig);         account.Register();     }     public void MakeCall(string destination)     {         var call = new Call(account);         CallParam callParam = new CallParam();         call.MakeCall(destination, callParam);     }     public void HangupCall(int callId)     {         var call = new Call(account, callId);         call.hangup();     }     public void Shutdown()     {         ep.libDestroy();     } } 

Program.cs

Now create the Program.cs file to interact with the user and get SIP details:

using System; class Program {     static void Main(string[] args)     {         Console.WriteLine("SIP Softphone Example");         var sipClient = new SIPClient();                  // Replace these with your credentials         string id = "sip:[email protected]";         string password = "yourpassword";         string domain = "sip:domain.com";         sipClient.Register(id, password, domain);                  Console.WriteLine("Press 'C' to make a call or 'E' to exit.");         while (true)         {             var input = Console.ReadKey();             if (input.KeyChar == 'C')             {                 Console.WriteLine(" Enter destination:");                 string destination = Console.ReadLine();                 sipClient.MakeCall(destination);             }             else if (input.KeyChar == 'E')             {                 sipClient.Shutdown();                 break;             }         }     } } 

Explanation of the Code

• SIPClient Class: Handles the PJSIP endpoint and methods for registering, making calls, and shutting down the library.
• Register Method: Takes user credentials and domain to register the SIP account.
• MakeCall Method: Establishes a call to a specified destination.
• HangupCall Method: Ends an ongoing call.
• Shutdown Method: Cleans up the resources when