However, I don't understand why other participants are signaled (BLF = red) when they are having a conversation
Usually a red signal means "busy", there's a Yealink on my phone.
You can customize the colors for Snom according to the instructions above.
Can it be that this function only works if the pbx and the snom are in the same network.
No, it should not affect
In MikoPBX hints are described in context
[internal-hints]
exten => 90,hint,Custom:90
exten => 2003,hint,Custom:2003
exten => 201,hint,PJSIP/201&PJSIP/201-WS&Custom:201