disagreement with tax man. What is the definition of a home based business? would Answering a cell phone Qualify ?

The business needs to be registered and operating out of your home. If the business is registered to a different address than your home...it is no longer a home based business even if you do all of your work at home.

Change the legal address.

If you are an answering service or call center support, possibly. If you are answer an occasional call from your workplace, no. Home office expenses for employees are no longer tax deductible.

>disagreement with tax man. 

How's that workin' out for ya?

You can disagree all you want but very few deductions are available for home based businesses. Not even 5% of your rent, power or anything.

< would Answering a cell phone Qualify ? >

no.  An answer would also depend where you are as tax laws vary around the world.

If your house is the physical address of the business, it's a home based business.

If all you do at your "office" is answer a cell phone, it's not going to count for a home based office tax deduction. 

No, it would not qualify.

This article has some practical guidance:

https://www.uschamber.com/co/run/finance/home-base...