If you do, I would recommend using a local company, if your city has one. There are two types, one that just makes a loud noise and one that alerts Police/Fire depts. I own a Private Security company, and we don't sell or service alarms. I do know people in town who do, and they are very good.
I am also a Private Investigator, and one of my clients is a lady in her 70s who is a widow. She is well off financially, and I had her get an alarm installed in her very expensive house. It was set up to alert her, via cellphone if the alarm went off. I knew what would happen in that case. She would not recognize the number, so she wouldn't answer it. I told her that when we were getting it installed, and she smiled and said, "Well, I guess we'll just set it up to call you next".

Uh, OK. I have keys to her house (she trusts me implicitly) so I guess that makes sense.
Later she was out of state, and we had a thunderstorm. Apparently lightning hit somewhere near the house and set off the fire alarm.I got a call at about 0500 hrs from the alarm company. Had to meet the fire department at the house, let them in, check for flames and re-set the alarm. Everything was fine. I told her she owed me dinner on that one.

The alarm worked VERY well, and she is very secure with it. It covers fire, break-in and I believe carbon monoxide. The company that services it is local, so I trust them, and that is what I recommend. I believe it cost about $1,500 to install, but this is a very large home.
Alarms are good, in some situations.
Jim