This sounds like Tomothy Zahn's story "Guardian Angel" that I read in Distant Friends And Others .
The story starts by talking about a guy whose company invented the Kuntz-Sinn force beam. Then they announced their latest breakthrough - invisibility. From the announcement:
"We are also today setting up Guardian Angels, Incorporated, a
business which will lease invisible bodyguards to members of the
public.Each person employing one of our Angels, as our guards will be called, will need only to wear a lightweight communicator-sensor device--" he held up a dark-green choker-like neckband-- "and our Angel will do the rest. Naturally, we'll begin our operation with an extensive trial
period. For this test, we'll be providing five hundred Angels, free of
charge, to selected residents of New York City."
The story follows one elderly woman who is selected to get an Angel, and a reporter who is trying to figure out what's really going on. As you said, the lives of the people with Angels are improved, and fake neckbands can be bought on the street. The big secret is that the "Angels" are actually using the force beams from a remote location.
From the conversation with the reporter at the end:
"Of course there's no invisible men" Draut shrugged. ...
"You've got people somewhere in the city using phased force beans,
right? Using the neckband sensors to aim the things?"
Draut nodded. "They operate from a handful of centers scattered
throughout the area. With sophisticated military targeting equipment,
of course, the beams can be most effective in simulating the actions
of an 'invisible man.' "
The owner of the company had grown up in a bad neighborhood, and wanted to use his money to help others in a similar situation.
"The so-called 'testing phase' is all there is to Guardian Angels,
Inc. The rest of the noise we've been making about it was just for
publicity purposes, to make sure everyone knew about it. ... For
decades the poor and elderly have been at the mercy of both criminals
and those who simply want to take out their frustrations on someone
else. No one's done anything about the problem because the government
can't afford it and there's no profit in it for anyone else. So okay.
I've got money I don't need, and I'm taking a crack at it."
