This is great advice but it also brings up a point; why do these systems require a local number in order to operate? It seems that a firmware upgrade could possibly decimate the op's business.

Edit: I want to point out that I am in no way, shape, or form saying that the op could not make a decent amount of money on this(though, 10/month is a touch steep for someone living in an apartment) but I was just curious why the manufacturers of such devices limit them in such a way.