Ozeki C# Clean SIP port

Question

I'am working on simple VoiceChat in C#. I am using Ozeki framework to make a call with user.

Everything works fine, but I have a problem when I want to make a call after call. After first call, I have to register my ip again using Ozeki method. But one of my port is still in use. So how can I clear port 5060 after call ?

This is method for register ip:

void Ozeki()
{
    softphone = SoftPhoneFactory.CreateSoftPhone(6000, 6200);

    microphone = Microphone.GetDefaultDevice();
    speaker = Speaker.GetDefaultDevice();
    mediaSender = new PhoneCallAudioSender();
    mediaReceiver = new PhoneCallAudioReceiver();
    connector = new MediaConnector();

    var config = new DirectIPPhoneLineConfig(local_ip, 5060);
    phoneLine = softphone.CreateDirectIPPhoneLine(config);
    phoneLine.RegistrationStateChanged += line_RegStateChanged;
    softphone.IncomingCall += softphone_IncomingCall;
    softphone.RegisterPhoneLine(phoneLine);
}

So again, how can I 'clear' port 5060?