Look at the prerequisites:

Visual Studio: This solution is based on a console softphone, so a new Visual C# Console Application is just enough.

VoIPSDK.dll: The necessary .dll file can be found on this website: http://www.voip-sip-sdk.com/. It should be added to your references.

Test phone: To test your application you can use any VoIP phone stat supports video calling (e.g. Bria softphone).

You need to create a softphone and a phone line object, then you need to specify the SIP account to be used for the phone line. These configurations are needed to be able to register to your PBX. After calling the RegisterPhoneLine method the regsitration procedure starts, and the application will indicates its status due to the mySoftphone_PhoneLineStateChanged method. The PhoneCallVideoSender and PhoneCallVideoReceiver objects are responsible for video handling. To handle the USB webcamera, the WebCamera object can be used. The CallType class is used to identify whether the call is a video or an audio call.

using System;
using Ozeki.Media.MediaHandlers;
using Ozeki.Media.MediaHandlers.Video;
using Ozeki.VoIP;
using Ozeki.VoIP.SDK;

namespace Video_call
{
    internal class Program
    {
        private static ISoftPhone softphone; // softphone object
        private static IPhoneLine phoneLine; // phoneline object
        private static IPhoneCall call;
        private static string numberToDial;
        private static MediaConnector mediaConnector;

        private static void Main(string[] args)
        {
            // Create a softphone object with RTP port range 5000-10000
            softphone = SoftPhoneFactory.CreateSoftPhone(5000, 10000);

            // SIP account registration data, (supplied by your VoIP service provider)
            var registrationRequired = true;
            var userName = "444";
            var displayName = "444";
            var authenticationId = "444";
            var registerPassword = "444";
            var domainHost = "192.168.115.25";
            var domainPort = 5060;

            var account = new SIPAccount(registrationRequired, displayName, userName, authenticationId, registerPassword, domainHost, domainPort);

            // Send SIP regitration request
            RegisterAccount(account);

            // Prevents the termination of the application
            Console.ReadLine();
        }

        static void RegisterAccount(SIPAccount account)
        {
            try
            {
                phoneLine = softphone.CreatePhoneLine(account);
                phoneLine.RegistrationStateChanged += sipAccount_RegStateChanged;
                softphone.RegisterPhoneLine(phoneLine);
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error during SIP registration: " + ex);
            }
        }

        private static void sipAccount_RegStateChanged(object sender, RegistrationStateChangedArgs e)
        {
            Console.WriteLine(e.State);
            if (e.State == RegState.RegistrationSucceeded)
                CreateCall();
        }

        static void CreateCall()
        {
            //numberToDial = "333";
            //call = softphone.CreateCallObject(phoneLine, numberToDial);
            var dialParams = new DialParameters("333");
            dialParams.CallType = CallType.AudioVideo;
            call = softphone.CreateCallObject(phoneLine, dialParams);


            mediaConnector = new MediaConnector();
            var phoneCallVideoSender = new PhoneCallVideoSender();
            var cam = WebCamera.GetDefaultDevice();

            if (cam != null)
            {
                cam.Start();
                mediaConnector.Connect(cam, phoneCallVideoSender);
            }

            phoneCallVideoSender.AttachToCall(call);

            call.CallStateChanged += call_CallStateChanged;
            call.Start();
        }

        static void call_CallStateChanged(object sender, CallStateChangedArgs e)
        {
            Console.WriteLine("\nCall state: {0}.", e.State);
        }
    }  
}