Yeah, scanning ranges of numbers for various things like Loops, or ANIs, or weird intercept recordings and such. Finding a test line or a test function isn't illegal either. Just dialing a number isn't illegal, and trying to understand how a complex, closed system works isn't illegal either. Some say that the true Phreak simply explores the phone system and doesn't necessarily exploit it. Others say exactly the opposite.
Rule of thumb is if it's on private property, yes, it's illegal trespass.
Simply to learn more about what you want to know. That's a Western Electric model 102, circa 1930. I do, however, have an old, beaten up Western Electric 302 that still works perfectly, and gets used quite often. Mostly to be original. Actually, one day I was thinking about putting up this page while getting out of the shower.