How select the best alert system for an elder living in an apartment (not a home or facility)?

Electrical plug in or through a phone? What does portability mean? If they say it can be plugged in in another location, is that truly going to work there as well as it does in the home or apartment? How do I find out if the monitoring is being done at a UL listed monitoring center owned by the company an elder is getting the service from? Help, please. I can't get beyond the sales pitches!

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