At my previous job, I had recruiters cold-call me at work. I assume that they either had numbers, or they went through the phone system and collected a bunch of names. I doubt they got that number from any references I gave out, because I never give my work number for that reason. Same goes with resumes.

One thing I found out is that some recruiters will do anything to fill positions that they have. I can't count the times that I've been asked by a recruiter if I knew other people who would be interested in a job I wasn't interested in.