I think I found something. I tried it on CIPC - SCCP, nothing was displayed on the Phone when the button was pressed. Testing with other phone models now.
Here are the instructions if someone else is also intrested:
Adding an IP Phone Service to a Phone Button
If you want to do so, you can assign the service to a phone button (a speed dial button) that is configured as a service URL.
By default, the IP phone service can display under the Directory, Message, or Services button/options on the phone, depending on your configuration in the IP Phone Services Configuration window, so you only need to add an IP phone service to a phone button if you want the service to display as a speed dial.
You can only perform this procedure for IP phone services that are not marked as enterprise subscriptions. Perform the following steps to add a service to a service URL button.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.