So I don’t have any specific insight to what’s available in the Netherlands
But I kind of feel like maybe you’re explaining what you’re looking for poorly
First some terminology
SIM and e-SIM are basically how your cellular service provider knows that your phone is connected to your account. The phone that has either that physical SIM card inserted, or that e-SIM data gets the calls, texts, data, etc. that are supposed to go to you. Take the SIM card out or change the e-SIM, and that phone no longer gets those calls, texts, and data. Put that same sim or e sim on another phone and it starts getting all those calls texts and data.
VoIP is Voice over Internet protocol, basically sending a phone call over the Internet instead of over phone lines. This might be from a computer, or from something that looks like a landline phone (or maybe even is a regular landline phone with some sort of adapter) or from a cell phone with a VoIP app installed. To use it from a cell phone you’d need to have either a WiFi connection, or a cellular data connection, and to have that cellular data connection you need to have either a sim or e-sim.
I don’t think there’s any VoIP provider that’s set up to just use your phone’s dialer and text app to directly handle calls and texts (though I could be wrong on that, I don’t try to keep up with all of the different types of phone services out there) everything would have to go through their app. If you want to do that, and you’re either ok having no cellular data and all of your calls, texts, and data use would have to go over WiFi, or if you keep paying for a cell plan (and the associated SIM/e-SIM) maybe either just a data plan with no talk/text, or a regular plan and you just don’t use the talk and text parts, then you just need to track down a VoIP provider, sign up for an account, and install their app on your phone.
If you want to transfer your actual phone number from your cell phone to a VoIP account, either to use on your cell phone through that VoIP app, from a computer, or from one of those landline VoIP devices, I don’t think that’s really a thing. If you just want calls to your cell to go to your VoIP phone number as well you’re looking for call forwarding.
You might also be getting tripped up with things like WiFi calling, VoLTE/VoNR (marked by some carriers with terms like “HD Voice”) which are things that are all going to be dependent on a regular cell carrier, not a specific VoIP company, and may depend a bit on their network infrastructure and what features your partic6 phone does or doesn’t support.