Inexpensive alerting system right here....
I have posted info in a few threads about this, but it seems to come up again and again. This is not a criticism of anyone asking the question because it is a valid question needing answers.
Many places have sales on FRS radios. I googled it just now and found prices for Audiovox as low as $23 a pair.
I bought a pair for $29.95 at Big 5 sporting Goods (local store).
The point is, if you look, you can find them, or PM me with your location -- in general, like Denver area or Dallas, TX) and I will find the sale for you if you have trouble with the internet.
Getting back to the FRS radio. IF you are careful when you shop, you should look for one with a "Drop In Charger", 2 radios, the radios should have VOX (Voice Activated transmission) and that is about it.
If you have a neighbor or someone living in your house or apartment complex, these radios transmit in FM (like your TV) and are easy to understand under most conditions. You can put this radio in their bedroom, living room, etc. and then you can wear the other small radio like a cell phone -- on your belt, in a pocket, or lay it on the kitchen counter. I have done all of these things. IF the person you are watching has the radio in their room and calls for you (with the radio set on VOX) -- you should hear a clear transmission of what they want. Or if they are having a medical problem.
The downside is --- this is NOT a TV camera. You cannot see that they have fallen out of bed or slipped on the bath room rug.
I don't have all the answers, but we have used our inexpensive Audiovox FRS/GMRS radios for a couple of years -- with our grandkids. If they cry or hollar for us, we are right there. No system is perfect.
I think they have simple cameras, with sound, that will transmit to a receiver in the house, which then puts the signal through a dedicated small TV. Sound and picture. I bought some X-10 cameras once. Used them outside for security. They were not really intended for outdoor use so they did not last but a year or so. I paid about $70 for them. Black and white, no sound.
If this person is mobile, all bets are off. The FRS/GMRS radio might be the best thing. If you call for your elderly parent and they don't answer, you would know to go check on them.
There are probably a lot of questions I did not answer, but I hope I have pointed some of you towards an inexpensive solution to many of the problems you may face.
Best wishes for you to find a solution.
Things could always get worse!! Sure enough, things got worse.