I have to confess that I don't understand the subject at all.
I use a two SIM card phone. SIM 2 is used for WiFi only but I only use it (PAYG £5 per 30 days) as a last resort. When hiking I carry a MiFi (£10 PAYG) as my main access to the internet and to date have never had a problem. Whenever possible I use public WiFi (pubs, restaurants, etc) to make my telephone calls on the MobileVoip app. I also use my SIM 1 PAYG exclusively as telephone money and will only use it when nothing else is available, £10 often lasts me months!
No idea if the following is relevant to the subject of VPN but when at home I use my PC to make my outgoing telephone calls, (the person receiving cannot identify me at all unless I choose to tell them) we never use our landline, this number is for incoming calls only.
For those who do know about these things, how does my method compare with VPN? Am I any less, or more vulnerable? If I'm more vulnerable, what am I more vulnerable to?