That’s right. In the way that most people use the term VoIP, a voice call with your MVNO/MNO is different from VoIP.
Hence
isn’t really right.
If you are genuinely using VoIP (over a mobile network) and you are using encryption then your MVNO/MNO has not much idea * what you are using the phone for, not much idea * what codec you are using, no idea what you are saying. Your MVNO/MNO does not have the keys needed to get involved with your VoIP call.
*Obviously your mobile provider sees what IP address and port you are communicating with, so if that’s sip.example.com on port 5061, and your mobile provider sees the average bandwidth that you are sending to that IP address, then the provider can make reasonable guesses.
On the other hand, if you are using a voice call (whether over 3G or 4G) then encryption is not relevant to your mobile provider - because the encryption endpoint is your MVNO/MNO and hence they have the keys and they know everything about the call - codec, content, …