I have come across people who inquired about how to get the phone number for a competitor that has just gone out of business.
Well If the phone service has already been canceled, it is not likely that you will be able to obtain that number. The phone company usually puts disconnected numbers into a state of inactivity for a minimum period of time (that can vary from provider to provider). Also telephone companies do not normally let you choose your number, and when they do it is considered a vanity number with additional monthly fees in most cases.
What you need to do so that you can obtain your competitors telephone number is contact the owner BEFORE the store is closed and have them transfer the bill into your name. You should ensure that the bill is up to date before having the bill transferred into your name. Of course the current business owner is not going to want to just give the number away, so you should be prepared to offer some form of compensation for the number.
I am unaware of any laws that prevent the transfer of a telephone number in this way, BUT the telephone companies do frown upon selling phone numbers, because the subscriber does not own the number, it's ownership remains with the telephone company.
once the bill is under your name then you can port the number to your current service provider, but I would recommend just finding the cheapest provider that offers free/unlimited call forwarding and just forward it to your main line.
Why would anyone want to do this? Well your competitor has a customer base, and possibly has placed advertisements in the yellow pages, magazines, newspapers and other forms of media. You can tap into these resources and potential new customers with minimal effort and expense.
Once the number is transferred to your name you can view my blog about how to port your number if you need additional help. http://nyphonejacks.blogspot.com/2010/08/lnp-local-number-portability.html