There are a lot of new wireless options out there now. I have an old fashioned wire and wireless combo system which I was considering to replace because of the cost and need for a contract. Companies like SimpliSafe (
https://simplisafe.com/ are very intriguing for me. Wireless, no contract, reasonable cost, many feature options and so forth. And you can install or relocate them yourself.
Some would even work inside a TD or CT.
You do need internet/cellphone access.
Hope this helps.
PS I was able to leverage my old system monitoring cost down by threatening to go with SimpliSafe.