Demand function estimation The model specification is:

$$D_{ij} = A + aP_{ij} + bY_{ij} + cN_{ij} + e_{ij}$$

where:

$i, j$ = $i$-th year, $j$-th month,
$D$= demand (outgoing minutes)
$P$ = average price of call,
$Y$ = average income per capita,
$N$ = number of base stations.

During the 5-year period under examination, a tax has been levied on telecom gross revenues for two years. Is it reasonable to include instrumental variable such as amount of revenue/profit which could have direct effect on price? If not, perhaps you have any other suggestions. 

Any guidelines would be very appreciated.
It appears that you suspect that the regressor "price" is endogenous, i.e. correlated with the error term, and that you consider what kind of instrument to use in order to tackle endogeneity.  

You think a possible instrument could be revenues or profits, because they are correlated with the price. Say, Revenues, denoted by $R$. But

$$R_{ij} = P_{ij}\cdot D_{ij} = P_{ij}\cdot(A + aP_{ij} + bY_{ij} + cN_{ij} + e_{ij})$$

and so Revenues are a function of the error term, therefore they will certainly be correlated with it, and same goes for profits (and this should be obvious since both depend on quantity and therefore on the error term).

In order to find an admissible instrument, you must first conclude about what are the variables that may be included in the error term, and are correlate with price. This will help you to search for admissible instruments. 

PS: I cannot see the connection of the tax mentioned with the issue of instrumental variables estimation.