Before you pick up the phone, consider the following: (1) what exactly will you be asking for; (2) just how long has it been since you last spoke to your former boss; and (3) will you be placing the confidentiality of your search at risk?
Networking is an amazing tool and, in this market, it is one of the best approaches a job seeker can (and should) take. However, you have to be smart and yes, cautious. First of all, contacting former colleagues and even a former boss is a great idea. But, if you haven't spoken to the person in years and have made no effort to keep in touch since you left the firm, it can be quite awkward. The importance of maintaining a network of colleagues and other professional contacts cannot be understated. But, for this tool to be effective, you have to keep those contacts ''fresh''. In other words, if you have not spoken to your former boss in years and suddenly contact him to ask about helping you get a job, that may be not be the best route to pursue.