Adrian ... I use a firm called "Get Help Now, Inc." They seemed the most flexible in accommodating my lack of speech. They are supposed to ask me only yes and no questions, but that doesn't help now, as nobody can even distinguish between yes and no anymore. But if they don't get a response they can understand, they notify 911.
They provided a lockbox with a key in it for the front door. The give the combination to responders.
It's a local firm ... or at least they have a local address. May be a franchise. I am very happy with them, and have used the button twice in the last couple of months for my husband. In both cases, they had an ambulance here within 15 minutes, and called later to make sure everything had been resolved.
Also, they give you forms to fill out about medical conditions, and a sticker for front door saying "look on refrigerator door." This being LA, I don't like notifying the passing public that there are two helpless people inside the house, so haven't used the sticker.
I think they are all pretty much the same. This is only my own experience but I'm happy with them.