To draw attention to this jack so emergency responders know where it is, you’d want “Firefighters: telephone.”  Or, you know, something less stilted, but approximately that.

To explain what this hole is to the average bystander so they don’t stick a pencil in it, you’d want “firefighters’ telephone” or even “firefighters’ telephone jack,” just to make it really clear.

While I’ll concede that perhaps not everybody needs “firefighter” to be a single word (although a quick Google shows me that many actual fire departments agree with me), this is at least bizarre, and probably incorrectly punctuated.

Thanks, Reporter #1!