There is always someone willing to take your job. Most won't have what it takes. But managers will use that to cow you into a shitty bargaining position.

Look I've been you. I took the crappy job because I figured that was the only one available. There are better companies out there. Just ask around. Get a few offers.

There is nothing normal about being on call with a 1-3 minute response time. I'm pretty sure it's illegal as well.