Not all of them, but enough that the question is one of the most common ones that lands in this archive.
There are men inside who genuinely value their relationship and are faithful through their entire sentence. That is real and it happens. But incarceration creates conditions that test people in ways that are hard to predict, and not everyone passes that test. The honest answer is that you already know which category your man falls into better than anyone outside your relationship does. Did he run around when he was on the streets? If the answer is yes, incarceration is not going to change that pattern. If he was solid before, there is more reason to trust the same.
The dynamic inside is also worth understanding. Inmates lose control over virtually everything in their lives. They cannot make decisions about where they live, what they eat, when they sleep, or what happens to the people they love on the outside. That loss of control creates its own kind of anxiety, and it often comes out as paranoia directed at the partner. If you do not answer the phone, he has already constructed a story about why. That is not an excuse for dishonesty, but it is a context for understanding why the accusations and the secrecy tend to run in both directions.
As for finding out who he is calling, getting visits, or receiving money from, that information is not available to you. The same privacy protections that exist on the outside apply inside. His call list, visitor log, and account transactions are his business unless he chooses to share them. No platform, no phone carrier, and no facility will give that information to a partner or spouse.
The only way to know is if he tells you. And whether he tells you says everything you actually need to know.