seclusion

Assalamu alaikumCan you tell me the Shariah definition of Khulwa/seclusion?Does it include telephone/email? Or does it refer to physical seclusion where a man and a non-mahram could have intimacy without being seen?Say for example you wanted to discuss business, dawah, work or  marriage with a woman, over telephone/email/sms/letter/fax, is that permissable (as no one would know what you are talking about)?