Just posting this if someone else runs into the same problems.
I recently upgraded an old asterisk installation from chan_sip to chan_pjsip.
I changed SIP/ to PJSIP/ in the hint-Lines in extensions.conf
Afterwards i verified, that “core show hint” in console showed the correct state of the phones.
Everything seemed to be good.
Sadly, the BLF LEDs on a Grandstream phone stopped working. They all showed “green” all the time. They should have turned red when a line is in a call.
The solution was simple. Updating the phone to the latest firmware fixed the problem instantly. I did not have to change anything else on Asterisk side.