No i have not tried without a SIM, and im not sure that is possible
When asking for the test you need to provide the number you're calling with, a SIM-less phone doesn't have a number.
Maybe there is an option for it if you ask specifcally for a SIML-less test, but i had to provide a number.
I gave them the information and the date i wanted and they responded with this:
Beste Jorijn,
De testtijdvakken zijn van 9:15 en 10:15 of 13:30 en 14:15.
Graag hoor ik van je welk testtijdvak je wilt gebruiken.
Translation:
Dear Jorijn,
The test time slots are 9:15 and 10:15 or 13:30 and 14:15.
Please let me know which test time slot you'd like to use.
I don't know how they got those slots, but pretty much yes, i gave them the info and they let me pick a slot.