It's mandatory on all new cars, but you can disable it yourself if you want to. You are also allowed to replace it with your own system, although I'm not sure why you would want to. The eCall system is heavily regulated, so it only triggers in the event of a serious crash (usually only when the airbags deploy) and then calls the emergency services on speakerphone, as well as sending your location and direction of travel (using GNSS). It works in all EU countries and the UK, and there is no subscription cost.
The cost to the manufacturer is the hardware and some tiny per-vehicle fee to the mobile networks. IoT SIMs start from cents per month for small amounts of data in single quantities, for consumers, so the deal that the manufacturer has is going to be much better.