ISDN and SIP trunks are two different independent options for how to connect with a service provider for public phone network services (PSTN). PSTN stand for Public Switched Telephony Network and is the common name for public phone network services, it is not a way to deliver the last mile to a customer premises, that would be SIP or ISDN.
Not to be rude or anything, but based on your questions it seems like you might lack some of the basic knowledge on how this works. There are very good material on how this works available on internet. Before embarking on any journey in this arena it is recommended to gather a basic understanding of the concept or that you reach out to a reputable system integrator with a Cisco Unified Communication specialization to get help with this.
There is a common misconception that a SIP trunk always equals use of internet. This is far from common in my experience, as for the most a SIP service from a service provider is delivered over a dedicated circuit that is not internet based. There are for sure services that rely on an internet connection, in those cases you’ll be needed to account for security, ie encryption, for your communication as your traffic would be carried on an open unprotected media that is susceptible to eavesdropping.
There is no such thing as using SIP over PSTN or using SIP over ISDN. As I wrote in my first answer PSTN is the public phone network that you connect to with for example SIP or ISDN. There are other options for the connection, like FXO and E&M. Also I never stated that you specifically said that SIP would equal use of internet, I merely wrote that it is a common misconception that there is a direct correlation between the two.
