As per your query, I have 1 query. It is reimbursement or allowance?
If it is Allowances it will be considered as part of Salary and is Taxable. If it is Reimbursement it can be exempted from Taxation.
Now coming to your main query for this example,
if salary is 14000/- and 3000/- is Mobile allowances thn total earning per month is 17000/- and so 204000/- per year, so he is out of scope of Tax.
As per 2016-17 Budget up to income of 250000/- the tax is Nil, and under Section 87A if individual is earning less than 500000/- is eligible for rebate of 2000/- on net payable amount of Tax.