Interface casting in C#?

namespace IVR.VoiceElementsEngine
{
    public interface IVoiceResource
    {
        void ClearDigitBuffer();
    }
}

namespace IVR.VoiceElementsEngine
{
    public class VoiceElementsResource:IVoiceResource
    {
        private readonly VoiceResource _voiceResource;
        public VoiceElementsResource(VoiceResource voiceResource)
        {
            _voiceResource = voiceResource;
        }
        
        public void ClearDigitBuffer()
        {
            _voiceResource.ClearDigitBuffer = true;
        }
    }
}

namespace IVR.VoiceElementsEngine.CallObjects
{
    public class VoiceElementsLine : ILine
    {
        public IVoiceResource IncomingVoiceResource { get; set; }
        
        public VoiceElementsLine()
        {
            
        }
        public VoiceElementsLine(IVoiceResource voiceElementsResource):this()
        {
            IncomingVoiceResource = voiceElementsResource;
        }
    }
}

namespace IVR.Tests
{
    public class VoiceElementsLineTest
    {
        [Fact]
        public void PlayPromptTextToSpeech_Should_Play_A_Text()
        {
            // ARRANGE
            var voiceResourceMock = new Mock<IVoiceResource>();
            var voiceResource = voiceResourceMock.Object;
            var line = new VoiceElementsLine(voiceResource);