If you are currently working, let the interviewer know and inform him that you will be resigning from there which may take a week or so to wrap up. Do not delay the call by long as this may give a wrong impression also do not convey that you will join on the next working day just to leave an impression when you have some chores left to be attended which will delay the process. Be honest and straight as most of the firms are familiar with the procedure of resignation and will grant the time to join in with them when you have been relieved from your duty from your last working place.