I guess Matrix would be your best option then. I use Schildichat as client, which is a fork of Element with some extras.
But if you can't get a plan, why not get a prepaid burner SIM? You can buy a prepaid card for minimal amount and you generally keep the number at least for a year, and you put in 5~10 euro each year you can keep it active endlessly.
A lot of things require a phone number. Here, the goverment needs you to have one, but also most workplaces and even the DHL. Getting a cheap trow-away sim isn't a bad option. Especially since pre-paid SIMs aren't connected to your name like those on a plan are.