Here is a sample of how to do so:
// add using statements
using RFCOMAPILib;
// Do some stuff here if needed...
// initialize your fax server api
faxAPI = new RFCOMAPILib.FaxServer();
// you can set this in the .config files if need be.
faxAPI.ServerName = "[YourFaxServerHere]";
faxAPI.Protocol = RFCOMAPILib.CommunicationProtocolType.cpSecTCPIP;
// since i am working with multiple users, i iterated through the users and retrieved their
// respective passwords (encrypted of course) but you can insert the string value here or
// set it in the .config file.
faxAPI.AuthorizationUserID = "[UserNameYouAreRetrievingImageFor]";
faxAPI.AuthorizationUserPassword = "[UserPassword]";
faxAPI.UseNTAuthentication = RFCOMAPILib.BoolType.False;
// Do some stuff here if needed...
// initialize fax object
RFCOMAPILib.Fax rfFax = (RFCOMAPILib.Fax)faxAPI.get_Fax([FaxHandleAsIntValueHere])
Hope this works for you!