One of the biggest downsides to non remote spyware for cell phones is you can only spy on newer cell phones less than 2 or 3 years old. That is a serious limitation.
Remote cell phone spyware can spy on pretty much any cell phone no matter how old that cell phone is. In fact the cell phone can be so old it doesn't even have a smart phone operating system like Android, Blackberry, IPhone, Windows Mobile, or Symbian.
HOWEVER - since the remote spyware gets installed to your cell phone your cell phone must have a smart phone operating system (there is always a catch isn't there?). My personal advice is if you don't have a smart phone get one. Chances are you want on anyway because of all the new fangled things a smart phone can do nowadays and almost all carriers let you get the latest and greatest smart phone free with a two year service plan.
The reason you can spy on old cell phones with a remote install cell phone spy is the spy software has to be compatible with the phone it is being installed to. Since remote spyware for cell phones get installed to YOUR cell phone it doesn't matter a bit how new or how old or how ugly the target cell phone is.
To start spying on the persons old and/or non smart phone is once the remote spy app is installed you:
And that is all there is to start spying on an older cell phone. At any time you wish you can terminate call monitoring for that specific cell phone and go through the steps above to start spying on a different cell phone.