Missionary Question

I hold the view that all Christians are missionaries but I've come across people who strongly oppose this idea. I suppose for me it comes down to what your definition of a missionary is and what their job description is. I believe that the task of spreading the good news of Salvation belongs to anyone who has believed in what Christ did for them on the cross and that the job description of a missionary is to spread this news. This is why I hold this view. Others have told me that a missionary is someone who works cross-culturally or in an overseas context. Personally I see 'location' as the least important part of the equasion. What do you think?
(this is a fav pic of my Dad, he and Mum are missionaries in PNG)