After repeated testing, I have to conclude that something strange is happening.
It works fine with an incoming call to 1 phone. (incoming rules; Inbound Route -> Extension)
When an incoming call has to be sent to a ring group, things go wrong. (incoming rules; Inbound Route -> Ring Group)
As "incomming rules" I set that Trunk COline1 (CO) should go to a Ring Group, group 720
When I now receive an incoming call, the LED (line 1) flashes and nothing else.
If i choose to send the inbound to a extension and i forward te extension to the Ring groups it does works ...
Why (doesn't) seem to be able to send a call directly to the Ring Group?