This week, I tested a product I could have used during that scare: the 5Star Responder from GreatCall Inc., a sort of portable OnStar. It’s a 1.8-ounce gadget with a speaker and microphone that clips onto a keychain or shirt. One large center button calls an always on-duty concierge, who knows information about each caller—like medications, preferred hospitals, emergency contacts and the caller’s location (using built-in GPS). Once the situation is assessed, a nurse can speak with the caller, emergency services can be dispatched or, in less urgent scenarios, the agent can stay on the line with the caller.

GreatCall’s 5Star Responder is one of the first truly portable emergency-call devices. It runs on Verizon’s network, giving it coverage anywhere a Verizon phone works. Other emergency-call devices like Medical Alert by LifeStation work in the home and are geared toward seniors who rarely go out.

The 5Star Responder has a speaker, microphone and a button that calls an agent or 911.

The 5Star Responder will be available Wednesday from GreatCall.com and in stores like Wal-Mart and Sears on Oct. 23. It’s $ 50, plus a $ 35 activation fee—or $ 25 if you sign up online. A $ 15 monthly service fee is applied, with additional family members paying $ 9 monthly. Also on Wednesday, 5Star Urgent Response will become available as a $ 15 iPhone app in Apple’s App Store, though the monthly service fees still apply. The website, MyGreatCall.com, where users set up their personal data, also launches.

When It’s Handy

Still, the 5Star Responder is yet another thing people will need to remember to plug in and charge (one charge lasts roughly three days in standby, the company estimates). It also felt noticeably heavy on my keychain. Yet 5Star Responder could really make a difference in situations where people don’t call 911 because they don’t believe they have an emergency, like not taking the symptoms of a heart attack seriously.

The 1.8 ounce device can fit in a small purse or clip onto a keychain.

The device isn’t just for seniors with health problems. Take a child who is too young to have a cellphone and gets separated from his family at a festival. When he presses the 5Star button, an agent determines his location and gets parents on a conference call with the child via the device’s speaker.

The device also could be good for a runner who doesn’t want to carry a heavy smartphone and collapses in pain during a run. Or a person walking in a sketchy neighborhood who wants the agent to talk to him until he gets to his car.

The device will call 911 directly if you hold down the call button for five seconds. If the Responder’s center button is pressed and a caller can’t respond, the agent will try to call the device back and an alternative phone number, such as an emergency contact, before finally dispatching emergency personnel.

Like the Real Deal

To test the 5Star Responder and a prereleased version of the iPhone app, I “suffered” through a variety of pretend scenarios. These included revisiting my above-described migraine, walking alone at night in an unsafe neighborhood, heart-attack-like symptoms and reporting a break-in at my house in the middle of the night.

I informed the agent on each call that this was a test case. They still acted like it was a real call, doing things like asking me to hold while they contacted paramedics, simulating the time it would take to dispatch emergency personnel.

Walking Alone

In one test, I told the call agent that I was walking in a neighborhood that didn’t feel safe, and he offered to call the police to come to my location. I told him I wanted to stay on the line with someone as I walked and he spoke with me for another five minutes until I got to my destination.

Once, I pressed the call button but hung up in the middle of the first ring.

Seconds later, the 5Star Responder device rang, and when I pressed the center button to answer, an agent told me he was calling back to make sure everything was safe and secure.

When I called to report someone in my house, the agent asked me to hold while she sent police. When she got back on the line, I asked her to connect me with my emergency-contact person, and she initiated a call to my husband.

When I called and described my migraine experience as if it was really happening, the agent suggested sending emergency-medical personnel, but I asked if I could speak to a nurse.

My agent transferred me to GreatCall’s LiveNurse service, which took about 40 seconds before that service’s operator transferred my call to an actual nurse. Before talking with me about my situation, the nurse asked me to spell my name and give my birth date.

This relatively lengthy process could be problematic in some scenarios. When I was suffering from my migraine, I could barely say a sentence on the phone with my husband because even the sound of his voice in my ear was excruciating.

In the end, the nurse suggested sending paramedics because I reported shallow breathing and an irregularly fast heartbeat.

Write to Katherine Boehret at katherine.boehret@wsj.com

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