You need to Google your local services, not buy one independently, as they are part of a telephone network which will be installed into your home.
The wrist or pendant alarm (whichever you prefer) will be part of the package and it will be linked via a modem added to your landline which will be linked directly to a registered care company who will answer the calls, if and when activated, 24 hours a day for any emergency.
You have to pay a monthly amount for this service, obviously, and the equipment is unobtrusive and can be installed in minutes and similarly can be removed easily.
If you fall, for example, and press your alarm the Careline operator can call your registered contacts, i.e. friend, family member, neighbour, etc., or call emergency services on your behalf.
For peace of mind and reassurance it is a fantastic and effective system, not just for you but also for your loved ones as we all fear somebody falling and not being able to get to a phone to summon help.
Most local councils will have details on their websites about the local providers.