public extern static bool SipSetCurrentIM(byte[] clsid);

Declare Function SipSetCurrentIM Lib "coredll.dll" (ByVal clsid As Byte()) As Boolean

public extern static bool SipSetCurrentIM(byte[] id);

bool success = SipSetCurrentIM(ClsidLargeKB.ToByteArray());

     Debug.WriteLine(string.Format("SipSetCurrentIM failed: error code {0}", error));