Recently, I got a call to move a phone system for a customer. It amazed me how nobody had a clue what they even had or what they were paying for. I knew more from memory than anybody in the business. Ok, to be fair I installed their phone system, but still.
They had a 4 line hunt group (rotary group for you old school types), a dedicated fax line, and 3 additional lines they weren't using for anything. One of those additional lines had DSL service on it they were not using either. They had business cable for Internet service. Three commercial phone lines with high cap commercial DSL was over $200 per month they were just throwing away. No, none of them were dedicated alarm lines. The alarm panels are on alternate communications.
This is a service you can offer your customers if you have the know how. I know a lot of us saw fly by night business consultants get customers to cancel phone lines they thought they weren't using a few years ago. Shortly thereafter their fire alarms (if they were setup right) and their burglar alarms quit reporting, and started displaying trouble conditions. Realistically with a commercial pots line running 40-60 per month there is not much reason to have an alarm system on one if the local AHJ approves other methods for fire and you have service in the location. Our local AHJ just says as long as it's a listed device for the use.
So, do a phone line review with your customer instead of letting some fly by night consultant do it and leave your alarm panels beeping. If you do it, you know your panel won't be left voiceless, you can save your customer some more money, and you may even be able to make a small up sell for alternate communication. The best part is if you do it then you know your alarm will still be working when you leave.