Under GSM it says Calling via 4G disabled; is this correct?

Yes, that’s possible. When using Octalarm Connect (via the SIM card provided), VoLTE (calling via 4G) is indeed disabled because it is not supported by KPN as yet.

If you use a proprietary SIM card, this setting depends on the telecom provider and/or type of subscription.

This is resolved as follows

Turn on Calling via 4G.

See Settings|Interfaces|GSM: Customise GSM settings for further explanation.