Most B-westerns of the 1930's to the 1950's followed well worn plot lines. There is a reason that they were called programmers since they often seemed to have been made by a program. They tended to be popular since they gave the audiences exactly what they wanted with out too much effort. As a rule I'm not a big fan of them simply because they are so cookie cutter like.
Cookie cutter or no they did allow for the rise of many heroes, Roy Rogers, Gene Autrey,John Wayne, Johnny Mack Brown rose to star status because through force or personality they could take a run of the mill plot and make it seem fresh. Here we have Johnny Mack Brown doing what he did best, giving us a better than average western tale for our hours enjoyment.
In Desert Phantom we have the story of a gun and ammo salesman, a seeming city slicker, getting hired by the pretty owner of a ranch to help uncover the reason that someone is trying to kill or drive off anyone who comes near the place. Its your standard issue plot but through Johnny Mack Brown's personality and better than average direction the film actual is an enjoyable western. While you may be able to put a group of suspects together its not instantly apparent who the headman is. Sure there are clues but there are enough red herrings to raise doubts.
Run of the mill or no I liked it and I stayed put all the way through to the end which isn't always the case with westerns from the period. (though admittedly I do prefer the films of Mack Brown who turned out consistently better films)
I give it 8 out of 10 on the program western scale. Your results may vary based upon your tolerance for these sort of things.