Volf argues that we should look at work not as a calling, but as an expression of our spiritual gifts:
In accordance with the plurality of charisms, there can be a plurality of employment or jobs without any of them being regarded theologically as inferior, or a mere "job on the side" (117).
In contrast to this is the view of work as a vocation, in which you feel called to one thing and stick to it. You might be miserable in your work, but if you find something else, then you might feel as if you are disobeying God.
I didn't get the impression that Volf was completely arguing against the concept of being called to so something, but rather that he was arguing more for being called - and able - to do many different things because of how we are gifted.
My husband has felt called to be in ministry as long as I've known him (so have I, so that works out well for us). Over the years, that call has been more defined as youth ministry. He gets along with adolescents pretty well, and he'll tell you (if you know him well enough) that he shares a brain with most adolescents. He enjoyed doing volunteer youth ministry, but the few times he's been the youth pastor haven't been quite as enjoyable (paid youth ministry involves a lot less working with kids and a lot more pacifying of parents). In a complete 180, he's now waiting tables in a fancy Asian place here in town. It's one of those jobs that people work typically as a stepping stone or a side job, but for him, it's neither. This is his real job, and it pays like a real job. Jeff's gifts lie in hospitality and helps - does it surprise you that he likes his job then? He's a great waiter; yes, I'm biased, but I've seen him in action. Oh, and other people will tell you the same thing, as he has regulars who ask for him to wait on them. Waiting tables would wear me out emotionally and spiritually, but I'm an extreme introvert (those of you who know me in real life are probably laughing at this, but think about when you first met me and how little I talked to you or looked you in the eye. And how quickly I escaped.) and people wear me out. For my husband, people are energizing. I'm extremely content to sit in an office all day long and not talk to another soul, but that would kill Jeff.
Jeff's worked a lot of jobs over the course of the 17 years I've known him, and the jobs he's been happiest at have been the ones that allow him to use his gifts to wait on others. He has a MBA, and he's happier and less stressed now (working what some would consider "not a real job") than he was when he was in a "real job."
For the Christian, ministry happens everywhere, and believers don't have to be pastors to do ministry. In fact, Jesus calls all of us to be ministers:
Go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20and teaching them to obey everything I have commanded you. (Matthew 28:19-20)
You might not be called to or gifted for liturgical work, but you are still expected to be minister to others. You can do this in any job.
When you base your life on being called to a certain job, it's stifling and can be guilt-inducing. And what happens if you are forced to resign or fired? Were you ever really called to that job, or did someone get it wrong (either the decision to hire or fire you)? The best thing about Volf's redefinition of this issue is that it's liberating. There's a freedom in doing work that uses the gifts you've been given, and there's a freedom in knowing that you can change jobs at any time and switch to using other gifts - without guilt.