Threema Calls without Play Services or microG

I’m using Threema with my /e/-device. I don’t have microG enabled. Threema is working with polling. Does someone know how I can use the calling feature? When someone is calling me it does not ring on my phone.

I read the FAQ from Threema:
There is stated: * To receive calls when Threema is in the background, Google Play Services are required.

Is it possible get this feature working? Does soneone has experience with that feature and know how to use it without Play Services?

Well… try enabling it?

No, this will not work. I don’t want to connect to GCM/FCM Services. I’m using Signal. There it works out of the box. So I thought it will work with Threema the same way.