You will need
  • Internet access, telephone.
Call the phonethat you have. Indeed, this method is the most logical and easiest. If you have a phone, why not call him? So, try to contact the firm for this phone.
If this phone actually belongs to that firm, accordingly, you will talk to her employee. He can ask anything that interests you on a given firm, including its location. This method, as mentioned above, is the most simple and easy.
Find all the necessary information on the Internet. This can be done, for example, simply typing a phone number in one of the search engines. As a result, if the Internet has any information about this company, the results of a search engine you'll see it.
The negative side of this method, compared with the first, is not a very high probability of success in search. Although a good result is not uncommon: often in sites with the theme of "navigation" you can stumble on such information.
Ask for help to the relevant sites on the Internet, specializing in the issue of paid information. These can be sites any mobile operators or just the navigation on a particular region.
Maybe you can help one of these ways, and maybe you have to try all these methods. But none of them guarantee success in the search. And you only need time, patience and a little luck.