If the softphone is blocked with the following error, you need to add the URL to the jtel portal as a trusted URL in the Salesforce settings.

Error: b.static.lightning.force.com/:1 Refused to frame 'https://rele....jtel.de/' because it violates the following Content Security Policy directive: "frame-src 'self' blob: mailto: *.force.com *.quip.com ....develop.my.salesforce.com *.vf.force.com https://*.arcgis.com".

Step-by-step guide

1. Start the setup in salesforce:
   
2. Search for 'Trusted URLs'.
   
3. Add a trusted URL by clicking the "New Trusted URL" button.
   
4. Set the following options. Use your own jtel Portal URL as the trusted URL.
   

The softphone will not be blocked with the above settings and the issue will be resolved.