Showing a camera image on Fritz!Fon (DECT)

Hello,

the manufacturer of the Fritz!Box routers (AVM) also produces a companion DECT phone device. It is able to show a camera image (jpg) from a URL configured in the router. Docs by AVM are here: https://en.avm.de/service/fritzbox/fritzbox-7490/knowledge-base/publication/show/1602_Setting-up-live-images-from-an-IP-camera-for-FRITZ-Fon/

I tried to link to a camera entity in HA via the REST API, but did not succeed. The curl example request in the docs is works, but probably the request by the router is different from what HA expects.

Any idea how to link the two ? Would be nice to have access to the image from the outdoor cam on the DECT phone, e.g. when the doorbell rings.

Lars