As a side note, the reasons I upgraded my system to a G2 was that it was about 14 years old, I had no higher level codes, a G2 is about £30, I could use all the existing parts except the actual panel, and with a G2 I can have full control of the panel over internet/phone app (by buying a £50 ethernet module. A few on here use the service for £1/month, Google selfmon for more info if it interests you). Also mine didn't have an external sounder/strobe, only an internal one. Lots of ADT ones are the same, they're designed to contact the ARC (alarm receiving centre) instead of making noise outside. Handy once you decide not to pay ADT £30/month (for virtually the same service Selfmon gives for the aforementioned quid) that the burglar can take his time in your house as long as he has ear muffs with him.
The install date on this system is 2000 so approaching 18 years old. We didn't choose to ADT monitoring purely because it's £30 a month to monitor a family house (with two big dogs) that is occupied 22 hours out of 24. For a £1/month I'd probably have done it. The person that was here before us was a widow so I can understand her wanting full monitoring, police and fire, totally different needs from us.
Over the course of the next few decades there is a huge chance of false alarms with dogs and as my children hit the age where they get their own door key.