Your terminology is way off which makes your questions a little tough to answer but i believe i have a handle and will take a stab at answering.

First off direct extension transfer takes place within the.voice mail when the caller is allowed to dial an extension and be transferred versus using a selector code such as 1 or 2 which is then programmed to ring an extension. I don't believe that applies here.

Next it is important to realize that all hunting comes from the phone company and not the phone system. You also don't want to forget that in order to get two calls to the system you need two lines. The last thing to recognize is that to handle calls differently they need to be in two separate hunt groups.

If i understand your situation you have two lines in a hunt group going to the auto attendant and a third line that rings extension 18 and then goes to the mailbox for extension 18 if unanswered? If so you would want to add a 4th line and tell the phone company to make line three hunt to line four and you will be all set.

Calls to line one or two will be handled as is. If 18 is busy on line 3 the call rolls to line 4 and goes to the voice mail for 18.