How do I configure my KPN SIP Trunk?

You can look up the settings by logging in on https://kpn.com/sip

It shows the right proxy server, username (which is the same as your SIP URI) and you can choose your own password. 

For more information like codecs, sample rates, etc. check the specifications on https://kpn.com/eigenmodem 

Hi ​@Ritban 

This type of SIP trunk configuration unfortunately falls a bit outside the normal support scope of KPN moderators on the community. We mainly help guide topics and assist with general KPN services, while advanced PBX and SIP integrations are usually considered custom third-party configurations.

That said, the community here is often very knowledgeable and people regularly jump in with practical experience and technical tips. In that regard, the information shared by ​@D4ALL  is definitely the right starting point.

You can retrieve the SIP details through the KPN SIP portal and the additional technical specifications are documented there aswell