A Couple of Questions

In the chapter titled "Everyone, Everywhere, Everything" of Rising to the Call, Os Guinness discusses the "Catholic distortion" and the "Protestant distortion" of the concept of "calling".  The "Catholic distortion" elevates the spiritual above the secular while the "Protestant distortion" stresses the importance of the secular at the expense of the spiritual (pages 26, 35).  In other words, the "Catholic distortion" values a life devoted to serving God by giving up all worldly things (think monks and nuns or even people who think in order to show true devotion to God they need to become pastors or missionaries).  On the other hand, the "Protestant distortion" interchanges "callings" and "vocations" with "occupations", "jobs", and "trades".  Well today there are thousands of corrupt and  even some evil jobs (the sex trade, for example), but does this mean that those who engage in those jobs are "called" to do so?  I think not.  My real question is, are we called to specific occupations, such as pastoring, parenthood, doctors, grocery check-out clerks, or custodians?  Or, are these jobs placed in our paths so we can come closer to fulfilling our true calling?  And if that is the case, how does someone with a "menial job" (our society would call groundskeeping menial) discern his or her true calling?