After gaining some reputation with the NCR a ranger ran up to me and delivered a NCR Emergency Radio to me, which can apparently be used to call for support and backup. However, it only works within "NCR Operational Area", and apparently Bitter Springs, ostensibly a NCR camp, isn't in that area. So what is the operational area of the NCR emergency radio in which I can call for help?
The range of the radio is somewhat hard to quantify - there's no easy guidelines, sadly.
You'll generally have the best results if you call for support near a major NCR base (such as Camp Golf or Camp McCarran), and it gets worse the closer you get to fully Legion controlled areas. Bitter Springs is sort of an outlier, as it's not well supported by the NCR, so I could see it being considered outside the range.
However, the radio is also somewhat glitchy and doesn't always work in areas where it should. I've had situations where I called for support, was told I was outside the operational range, took a few steps further away from any NCR locations, then called again and got support. If at first you don't succeed....
Additionally, sometimes the support never comes - the supplies locker can sometimes not be where it's supposed to be, or the NPC support never makes it to your location.
It also doesn't work in any areas added by expansion packs, as the NCR doesn't operate in these external areas.