I have a friend who has been teaching college-level math (e.g., all levels of calculus) 
for about 4 years, although all of his education, including his Ph.D., was in engineering. 
Now he is considering applying for tenure track jobs at teaching schools and he is bemoaning the fact that nearly all the ads he sees include "Ph.D. in mathematics", rather than "Ph.D. in mathematics or related area" as a qualification. This raised a question which I don't have the experience to answer for him: are these ads, for tenure-track positions at schools that focus more on teaching, generally so serious about the Ph.D. in math that they would dismiss an application from someone with a Ph.D. in engineering instead of math? 

While there are other aspects of his application which are more important (he gets fantastic teaching evaluations and quickly learns how to use new educational technology), 
the only thing I'm wondering about on this friend's behalf is the Ph.D.-not-in-math issue 
and whether it's a deal breaker.

(Of course you may ask how he got his first job teaching math and why he can't do the same thing he did then. His first teaching job was short-term, not tenure track, and his hiring had some idiosyncratic features that are unlikely to repeat themselves for the next teaching job.)