It is always better to follow with a resume than to lead with it. What this means is that contacting and connecting with a prospective employer before handing over your resume will go much further in getting your resume attention. Obtaining phone numbers and even the name of a hiring manager is a relatively simple process for the job seeker that takes the time to do it. Phone directories are available on CD ROM from just about any software retailer and many companies make offline contact information available on their web site. If all else fails, call the phone company.
Companies are generally very visible.
If you are responding to an ad that does not list a phone number or company name, there is frankly not much you can do. But if you can find the employer, take a proactive approach to preparing the way for your resume. Sending a letter, conversing over the phone and especially holding a networking meeting with a prospective employer allows the job seeker to build interest and generate something memorable. When your resume sits in a stack of dozens of other resumes it is difficult to be distinguished. By connecting with an employer before sending the resume you stand a much better chance of having your resume reviewed.