Since the CPS I used was increasing the subscription cost to above zero, the the price difference to KPN was too large, so I wanted to switch back and needed the KPN CPS code.
From [WayBack] Het verschil tussen WLR en CPS I found out the CPS code for KPN is 1655.
Then I called 0800-1273 to set it back and then verified it was set back for all destinations on all ISDN numbers.
It’s time to research how to move away from ISDN anyway, but that’s a different story.
–jeroen