Ask to your provider if needs an special header to send the CID
I have been in touch with our provider and after changing some more things on their side it started to work.
Ofcourse I am satisfied it's finally working but I also would like to apologize for the wasted time since it ended up being a provider issue after all.
By default, it seems VitalPBX sends the "extension" as callerID.
VoipPlanet (and perhaps fieber.nl) will show NO callerid if the CID doesn't match the CID linked with the trunk.
With VoipPlanet I had to rewrite the CID using the international telephone number notation:
31NNXXXXXXX *eg. 31851234567 for Dutch number 085-1234567
I didn't have to, but you may have to the put the "+" in front of the country number.
The nice thing about VoipPlanet I believe, is they could link several CID to your trunk, so you could choose your outbound CID if you e.g. are using one PBX for several company locations.
It actually sends the external CID value of the extension that usually is the same extension number when is not configured with a different value.
If you want to learn more about VitalPBX environment, tips, and tricks, you may attend our online training course: https://vitalpbx.org/en/vitalpbx-online-training-course/